In 2004, Valve released the sequel to their revolutionary and critically acclaimed first game Half-Life, Half-Life 2, which itself broke new ground on many fronts in its graphics engine, physics system, and its online distribution/updating system.Half-Life 2 starts off about twenty years after the ending of the original game, as the mysterious G-Man uses his time and space bending powers to place Gordon Freeman on a train headed for City 17, some undisclosed place in Eastern-Europe. Gordon soon finds out that Earth has been invaded and conquered by a powerful alien force known as the Combine, which apparently detected the Black Mesa incident and took advantage of it to enslave humanity and add Earth to its collection of assimilated worlds. Having attained an almost messianic status among the fledgling resistance forces thanks to his actions in Black Mesa, Gordon's attempts to survive and eventually strike back against the Combine provoke a full-scale revolution. In this, he is aided by the Vortigaunts, an alien race whose enslavement he broke by his destruction of the Nihilanth; members of the Resistance, including several former Black Mesa scientists; and Action Girl Alyx Vance, who accompanies Freeman throughout much of the game as a fully-qualified AI partner. He is opposed by the vile Dr. Breen, former administrator of Black Mesa and now self-styled overlord of humanity.The game has to date been followed by two sequels, called "Episodes". In lieu of a full-out Half Life 3, these shorter games were intended to cut down on the development time that a full sequel would require, but it hasn't worked out that way.The first two Episodes chronicle Alyx and Gordon's escape from City 17 immediately after the events of the main game, and the rebels' continuous struggle against the remaining Combine forces, who are seemingly controlled by the mysterious "Advisors". The games also further develop the relationship between the two protagonists. Episode One was released in 2006, and Episode Two was released in 2007 alongside Portal and Team Fortress 2 in The Orange Box. However, as of 2014 there has been no word as to when or if Episode Three, intended to complete the Story Arc and resolve the cliffhanger that ends Episode 2, will be released, placing it in the same category of perceived Vapor Ware as the infamous Duke Nukem Forever.Half-Life 2 and its sequels are remembered for setting new standards for A.I. NPC companion characters (Alyx in this case), later reflected in games such as Bioshock Infinite.
Half-Life 2 contains examples of:
Abandoned Hospital: One of the settings in Episode One, which appears to have been taken over by the Combine and, from the looks of things, is in the process of being abandoned again.
Abandoned Mine: In Episode 2, one of these holds an antlion nest and a rebel outpost.
Abnormal Ammo: The crossbow in Half-Life 2 shoots red-hot metal bars normally used to reinforce concrete.
Any object picked up with the gravity gun can be launched as a lethal projectile as well, such as circular saw blades in Ravenholm.
Part of this is down to the fact the G-Man is probably not human at all and merely using a human host body or projection. The most blatant case of this is at the beginning of Half-Life 2 is abetted by the fact that Gordon's temporal rate is seemingly being tweaked back out of slow-time suspension while G-Man is speaking to him.
All Animals Are Dogs: D0G himself, whose morphology is (at least by now, after all the modification Alyx has been doing to him) much closer to that of a great ape, but still retains the doglike behavior he had programmed into him.
And I Must Scream: It is implied in-game that headcrab zombies are fully conscious and aware of their situation. Also, if you play their audio backwards, it becomes rather... disturbing.
Antepiece: Valve try to make their level designs fun and intricate but as accessible as possible. So for one of the more complex "puzzles" in the game, there is an "antepiece" - a preceding engagement that will help you out with what is to come. In the puzzle, there are a bunch of barnacles on the ceiling with their tongues hanging down. There's an explosive barrel near you. If you pass the barrel to a tongue, the barrel will be pulled up to the ceiling, and you can shoot it, and it will explode near the barnacles, and they will all die in a satisfying way. That's the puzzle, and Valve wanted to clarify before it "you can carry barrels" and "barnacles will pick up barrels". So just prior to entering this room, there's a part where you must elbow your way through a bunch of (nonexplosive) barrels. When you do this, one of them will fall down a slope and slide into a small, non-threatening group of barnacles where, in full view of the player, it will be picked up by a tongue. This is described in more detail, with pictures, half way through this article: 
Antlion Monster: The monster actually subverts the trope. It's called an antlion but actually has little in common with actual antlions; it's called that because it's a lot like an ant, but is the size and ferocity of a lion.
Artifact Mook: Headcrabs appear at first mostly in places where the Combine have specifically used them as a biological weapon, and they're usually accompanied by the remains of their artillery shell cages. After the Ravenholm level however, they become more-or-less just Goddamned Bats ready to pop out of any vaguely abandoned area.
Artificial Brilliance: Half-Life 2 often appears to have bad AI - enemies that just stand there shooting at you. However, the AI is actually pretty good - it's the level design which often doesn't make the most of it. Half-Life 2 soldiers are programmed to take cover and look after one another, but in the tight corridors that a lot of the game takes place in, this can't be well demonstrated. This video demonstrates the AI in action.
Hunters are even better when it comes to the AI department - they suspiciously often take pathfinding decisions that result in at least one of the Hunters in the pack ending up ahead of a fleeing prey, cutting off the escape route. And if you try running them over with your vehicle, they'll act as though they're about to be hit, and then jump to the side at the last minute.
Artificial Stupidity: Resistance fighters in Half-Life 2 are actually pretty good A.I.-wise (they strafe/use cover, verbally recognize different enemy types (i.e. "Combine!" "Zombies!"), and are smart enough to back away from melee enemies while firing), but they do have an annoying tendency to charge straight into sniper rifle fire, cluster around the player instead of giving Gordon some breathing space, and cannot be told to "wait" in a safe position for more than several seconds.
There is also the infamous "barrel trick" in Half-Life 2, where a certain Metrocop gunner in the beginning of the Water Hazard chapter can't see you if you can't see him (say, if you carry a barrel or even a paint can so that it obstructs your view of him).
At the beginning of Episode One, Dog shaking his head after Alyx asked if he did the math was originally a bug. It was funny, so they decided to Throw It In.
One section in Episode Two features a very audible zombie inside a dumpster. During development, playtesters commonly lobbed grenades into it, thinking they were being clever. At least one time, a physics glitch shot it right back out, making it look like the zombie had done it. The programmers thought it was hilarious and decided to re-code the sequence so that it would happen on purpose every time.
Badass Bookworm: Gordon himself. Before the whole mess started at Black Mesa, he was a valued but decidedly junior theoretical physicist with an interest in athletics, often picked to do the grunt work as he was talented with a HEV suit. (This is lampshaded in Red Letter Day by Barney early on in HL 2.) From then on, he proves himself to be a Determinator, chewing through anything that gets in the way and being noted for how unlikely his survival talents are by everyone from the G-Man to Breen.
Whether or not Grigori is actually an ordained priest was intentionally left ambiguous: he's one hundred percent Laughing Mad. The crude but effective traps he designed and built all over Ravenholm offer little evidence either way — priests have historically had enough education and free time to do a lot of tinkering and inventing.
The Beastmaster: Gordon Freeman is a type 2 once he gets Pheropods. Unfortunately, not every type of Antlion will listen to your orders.
Bittersweet Ending: Episode Two. Yes, we defeated the Combine assault on White Forest, and closed the superportal. But Eli is killed by an Advisor in the Resistance's moment of triumph, his last words desperately telling his daughter to look away. Alyx and Gordon survive only due to D0G's counterattack, and the game fades to black as Alyx cries over her father's body.
Blinding Camera Flash: The scanner enemies do this if you're looking at them when they photograph you (this is pretty much the only hazard they pose, apart from occasionally giving your position away to a strider).
Revisited in a much deadlier fashion in the Episodes as the Advisors' mental attack is displayed onscreen as a visual distortion, including the Bittersweet Ending above.
Border Patrol: Leeches will attack if you wander too far into the sea, and unlike the first game they're unkillable.
But Not Too Black: With a black father and a Malaysian mother, Alyx is at best Ambiguously Brown herself. Gets particularly jarring if one looks at a picture of Jamil Mullen (whose likeness Alyx was modeled on), who could credibly pass for Eli's daughter. Presumably Alyx really takes after her mother.
Given the existence of Quincy Jones's real life daughter Rashida Jones (and knowledge of the many ways interracial genetics can manifest) Alyx's appearance becomes significantly less jarring.
Calling Shotgun: Alyx, before she and Gordon Freeman go for a ride in a car.
The Can Kicked Him: There's an achievement in Half-Life 2 for killing an enemy with a toilet.
Car Fu: Thanks to the Havok physics engine and the Gravity Gun, cars are deadly weapons. The driving sequences in Half-Life 2 and Episode 2 both feature ramming as instant-kills on most enemies, and the Gravity Gun can punt abandoned cars for major damage. There's also a sequence where you can use a magnetic crane to smash Combine soldiers with the car. The Antlion Guardians get revenge, though; their AI is programmed to deliberately heave cars at you.
Much like Flushed, the Achievement for killing something with a toilet in the main game, Episode One has an achievement for killing a Zombine with its own grenade. Do this by snatching it out of the kamikaze's hand with the Gravity Gun and flinging it right back at him.
Climactic Elevator Ride: The game has Gordon and Alyx ride up an elevator before the final battle. Alyx takes this time to say a few things.
The Computer Is a Cheating Bastard: Inverted. Your go-to medium range weapon, the Pulse Rifle, does almost 3 times as much damage in your hands as in an enemy's for no real reason, and its secondary fire instantly vaporizes anything but you and plot-important friendlies. In that case, it only does a negligible 15 damage.
Cool Boat: A fan driven boat that can run on land. Mounted with an infinite ammo gun that was ripped off an attack helicopter.
The Mudskipper is specifically equipped with it to take down the one(s) that caused such grief during the earlier parts of Water Hazard. As the Railroad Rebel says, "I like to bring a little irony to a firefight." If you don't monkey with it, the recharge and firing periods are the same as the hunter chopper, but you do a lot more damage to things with it than is done to you, just as with the AR2 in the above listing.
Cool Car: Muscle Car. Then there's the original car, which is an extremely fast dune buggy with nitrous oxide tanks and a laserTau Particle Gun on top.
Crapsack World: Earth in the hands of the Combine; overrun with such otherworldly horrors as antlions, headcrabs, & barnacles, what few cities that remain are barely-functioning ruins, and humans seem to be on the brink of extinction.
Crowbar Combatant: In Half-Life 2, it's the weapon Barney gives you before you make your run for the canals (and implied to be the same one Gordon dropped in Black Mesa).
In Episode One, he gives you another and lampshades that he's starting to run low on them, and Alyx mentions it in reference to a victory he just achieved.
In Episode Two, the G-Man cites its attack as what the Vortigaunts knew of humanity before Gordon freed them by killing the Nihilanth.
Crouching Moron, Hidden Badass: Aww, see D0G follow Alyx like a sweet puppy. See D0G play catch with Gordon. See D0G hurl himself onto a strider, punch through its chassis, and rip its goddamned brains out.
Cyber Cyclops: The Elite Combine in white armor have one, glowing red eye.
Degraded Boss: The Striders get significantly easier to destroy as you move through the games. Justified somewhat in that you're getting better weapons to deal with them. Completely averted in Episode One, where a Strider is not only the final boss, but easily one of the most frustrating things to fight in the entire series thanks to its near-perfect aim and destroyable cover.
In Episode Two, the Antlion Guardian, inside the tunnels, is ridiculously dangerous, has infinite health and is covered in a very potent neurotoxin. But when you actually have to kill it, the invulnerability and neurotoxin power has been removed, essentially turning it into a palette-swapped Guard.
Drought Level of Doom: Ravenholm is a zombie-infested ruin without much ammo. The player is, however, provided a wide range of sawblades, explosives and environmental traps to use against the zombies instead.
Dual Boss: The first boss fight in Episode 2 is a fight against a pair of Antlion Guards (technically a Guard and a Guardian, which is just a palette swapped Guard) and their normal antlions minions. They don't hesitate to attack you at the same time, but you've got a healthy supply of SMG and Shotgun ammo, a safe spot were they can't strike you, and several exploding barrels lying around the valley...
DVD Commentary: An interactive example: in Episode One and Two and the spinoff Lost Coast, there are commentary bubble icons the player can "use" to listen to information about making the games.
Egopolis: New Little Odessa, the Resistance base run by Odessa Cubbage.
Eleventh Hour Superpower: Your Gravity Gun gets a massive upgrade and allows you to pretty much pwn your way into the heavily guarded Citadel.
This returns for the beginning of Episode One.
The Empire: The Combine are a clear example. Their plans for Earth? 1) Invade; this includes most humans dying. 2) Remaining population becomes either a Stalker, a Transhuman soldier, or dead. 3) Resources, including oceans and atmosphere, get transferred off-planet.
Episodic Game: Valve decided to continue the story of Half-Life 2 in episodes, when the concept was new (And it still is a fairly recent one) However; it turned out to be averted, since the point of Episodic Gameplay is to not keep people waiting for a long while in between episodes. Episode Two was criticised for this by Zero Punctuation, but a lot of people shrugged it off because it was at least released as part of a bundle that also included the long-in-development Team Fortress 2. Episode Three, as of 2013, appears to still be in development. That's right...it's been in development for more than five years. (And was already in development while they developed Episode Two, as said in the commentary node for the gunship battle.) It's almost been jokingly called "Valve's Duke Nukem Forever".
Expository Gameplay Limitation: Despite popularizing the trend for never breaking gameplay in the original instalment, this game and its expansions feature a subtle example of this trope: whenever Freeman is facing a friendly NPC, often while the NPC in question provides exposition, he will automatically lower his weapon, preventing the player from firing.
Flechette Storm: In Episode 2, the Hunters have an automatic cannon that launches explosive flechettes as their main weapon, and they often come in packs, so having several Hunters shoot at you at the same time creates this effect.
Foreshadowing: "That's the old passage to Ravenholm...we don't go there anymore." Guess where you're headed?
The same applies to pretty much any location or route that is mentioned by name early in the game. Bonus points if the person mentioning it says the place or route is dangerous.
Also in the comment that it is spawning season for the Antlions. Very possibly in G-Man's warning to Eli (by way of his daughter) to prepare. White Forest's successful shutdown of the superportal led directly to Eli's death, after all.
Gaia's Lament: Earth has been ravaged and drained of its resources. Water especially. Check out the designs of all the levels that feature waterways or oceans, there's plenty of clues in the scenery that the water level used to be about 5 metres higher than it is now. The amount of water that's been stolen to make global water levels drop that much is truly mind-boggling.
Give Geeks a Chance: Alyx certainly seems willing to, but admittedly Gordon is a pretty badass geek, to the point that he renders her almost speechless taking down a Gunship singlehandedly in Episode One. She gushes for a good clip before suggesting he whack the corpse with the crowbar a bit just to be sure it's dead.
Goddamn Bats: invoked The commentary for the episodes notes that black headcrabs were found to be perfect for scaring the player, without actually being a threat, as they will bring Gordon's HP to One, but can't kill him and his HP is restored to what it was before rather quickly.
Guide Dang It: Did you know Gordon can sprint? If so, then somebody told you, because the game never ever brings it up. The developers correct this in Episode 1, where there's a segment in the Citadel designed specifically to teach you how to run.
Hair Decorations: Doctor Judith Mossman has a large plain hair clip at the back of her head.
Happy Ending Override: Half-Life ends with Gordon Freeman successfully killing the alien being that prevented the scientists on Earth from sealing the portal that spewed forth endless hordes of alien invaders. He gets captured by the G-Man and put into a freezer, but at least Earth is safe. About two decades later, Gordon is brought back to Earth, only to learn that the alien being he killed was just desperate to allow its own people to escape from an even scarier and more powerful alien invasion of its home dimension. With Freeman taking care of their leader, the Combine quickly had the alien world conquered and continued its campaign by invading Earth as well.
Blue Shift had the only unambiguously happy ending of the first Half-Life games, with Barney and his scientist comrades escaping Black Mesa; and then the Combine invaded.
Heel-Face Turn: It is not made clear whether Dr. Judith Mossman was genuinely working with the Combine or was a Reverse Mole, but they change sides again just in time to rescue Gordon, Eli, and Alyx from Dr. Breen in the Citadel.
Heroic Sacrifice: Father Grigori, possibly, as towards the end of Chapter 6 in Half-Life 2, he tells you to move on while he buys some time. It's possible that he was overrun by Headcrab Zombies and died, but given how good he is at killing Zombies, it's also possible that he lived. If you stick around to watch the battle, he adamantly refuses to die in front of you no matter how long you linger.
Hide Your Children: Justified; the Combine have been suppressing reproduction for almost 20 years by the time of Half-Life 2.
Hoist by His Own Petard: You can one-shot Hunters by catching their darts with a physics object and shooting it at them.
Hold the Line: Numerous instances where you have to hold off foes to wait for an elevator or some such.
The final battle in Episode Two is this, killing Striders before they can fire upon and destroy White Forest. Optionally you can keep them from destroying the smaller buildings as well for an achievement.
Hollywood Tactics: The Combine suffer a huge dose of this in one battle. When word first reaches them that Gordon has come back, they mobilize hundreds of Civil Protection officers, Armored Personnel Carriers, and Hunter-Choppers to go after him. They're all slaughtered. Then Gordon survives through Ravenholm and starts wiping out several Combine Overwatch checkpoints along the coast. The Combine then got solid intel somehow about his location at Lighthouse Point. So what do they do? Send a bunch of planes to drop lots of bombs from the safety of the air, where he can't touch them? Nope. Instead they send a Synth gunship and several squads of troops. Can you guess what happens next?
Hostile Terraforming: The game in its released form contains hints of this - most notably, the Combine drain the Earth's oceans and use Xen species such as Headcrabs and Leeches as biological weapons. The original beta however went much further - a key location in the game was the "Air Exchange", which replaced the air with noxious gases breathable by the Combine, and which meant that all human characters in the game had to wear gas masks.
Gordon Freeman, who certainly becomes a Messiah-type figure to the ragtag human rebellion in City 17, and the Lambda symbol itself becomes a common symbol to denote rebels and the caches they hide around the city and surrounding countryside.
Also Father Grigori's shoes. Red sneakers were a fashion taken up by freedom-loving Eastern Europeans during the glasnost era, and Father Grigori looks to have been one of those rakish youths back in the day...
Idiot Ball: Gordon Freeman climbed inside of a moving metal coffin in the Citadel just because he had no clue where else to go. Twice.
Improvised Weapon: The Gravity Gun, which turns virtually anything into a weapon: Chairs, crates, tables, barrels (exploding and non-exploding varieties), benches, radiators, armoires, TVs, tires, bicycles, cars, people... The gun itself is an example as well, it was previously used for heavy lifting and construction.
Happens at the end of Black Mesa East, when the Combine shells the hideout and causes a cave-in.
Recurs during Anticitizen One, when D0G opens a Combine barrier then is carried off by a troop transport, then again when Alyx later scales a building to spot the means for much less monkey-like Gordon to progress by and is promptly captured by a Combine patrol.
Ironic Echo: Midway through the game, Breen dismissively ends his conversation with Dr. Mossman by saying "So sorry Judith, I'm all out of time." Later, when she turns on him and he's pleading with her, she says "I'm sorry Wallace, you're all out of time."
Irrelevant Importance: The buggy, which is required to cross a certain bridge. If you pass that bridge and destroy the buggy (e.g. have it go over a cliff or get it tapped by the train), you fail a critical objective. However, abandoning the buggy at that point and proceeding on foot still lets you complete the game.
especially when a Combine Dropship hauls it away right after your victory at Lighthouse Point.
It's Probably Nothing: The people worry about the mind-controlling water in Half-Life 2, but have no choice (unless they live outside the city, in which case they can just throw together a purifier). One of the scientists uses this exact phrase just before the experiment at the beginning of the first game. Yeah, that one that tears a hole in reality and causes all hell to break loose.
Lampshade Hanging: A quarter of the Vortigaunt's lines in Episode 2 are poking fun at the game's own use of tropes, such as the prevalence of Broken Bridges or Gordon's habit of falling into pits. Examples include, "Pity the generator that requires a Vortigaunt to operate," "No pit would be complete without a Freeman climbing out of it," and, "What next in the parade of constant obstacles?"
Laser Sight: The Combine Snipers, the Rocket Launcher, and the Combine Autogun in Episode Two. The rocket launcher example is justified, because it fires laser guided missiles. The others are just so the player can be Crosshair Aware.
Leaning on the Fourth Wall: The Vortigaunts tend to do this, if you stick around and listen to them enough. In Episode Two, various ones spend a great deal of time remarking on the ridiculous number of convenient obstacles put in Gordon's way. (see the Lampshading entry above for specific examples) Even in Half-Life 2, however, they did a little leaning, mentioning "the eyes within your own, the minds within your mind". In context, it seems to refer to G-man and whoever he's working for but it also seems to be the Vortigaunts subtly pointing out that Gordon is and has been controlled by thousands of different players.
Vortigaunt: How many are there in you? Whose hopes and dreams do you encompass?
Left Hanging: Episode 2 ends on a cliffhanger, with Alyx's father apparently dead and she and Gordon at the mercy of the villains, which may be left unresolved if the long-delayed Episode 3 (seven years and counting as of 2014) remains Vapor Ware.
Lighthouse Point: The Trope Namer. At one point Gordon visits a lighthouse called Lighthouse Point and has to fight off a gunship with the rocket launcher ammo stored inside.
Hunters are crazy fast and crazy tough (able to eat almost 2 mags of automatic weapons fire before dying), and have a pretty mean automatic cannon for a nose. Antlion Guards are also fast and tough, but their dependence on melee ramming makes them more of a Bullfight Boss.
The Hunter Choppers, who instead of being Heavies like the Hunters or Boss in Mook Clothing like the Guards, are straight bosses themselves. They are far superior to any other vehicle or Synth the combine can field. They fly, move extremely fast, can eat up much more damage than an APC, and come armed with an unerringly accurate and powerful pulse machine gun and a seemingly unlimited supply of mines. Fortunately, they're rare (only two are encountered in the whole series thus far, three if you count the one in the playable cut beta episode "Lost Coast" ) and you're fortunate enough to encounter while they're on their own and you've already killed all mooks prior to fighting them.
Madness Mantra: A 'passenger' at the very start at the game keeps repeating "...they're always departing but they never arrive... and the ones that do arrive, they-they never leave... you never see them go... they're always full... no one ever gets on... but they're always... they're always departing but they never arrive..."
Magic by Any Other Name: The Vortigaunts' powers are never named as magic and sometimes get a Technobabble explanation, but they involve chanting and waving their hands and let them shoot lightning, heal mortal wounds, perform Mental Fusion and whatever else the plot needs.
Malevolent Architecture: Combine architecture is designed to slowly expand and consume anything that gets in its way. Including you in Nova Prospekt. Better move quickly.
Monster Munch: In the second level, the Combine start headcrab-shelling the outskirts of City 17 to try and kill Gordon. At one point you get to watch as a headcrab latches on to a civilian's head and turns it into a zombie (or Mercy Kill him before he gets assimilated, whichever floats your boat).
Never Trust a Trailer: One of the trailers for Episode 2 Showed Alyx hanging from a bridge. The following scene of a Vortigaunt carrying her implied that she fell to her death.
Nice Job Breaking It, Hero: In Episode 2, Gordon has to make his way through an Antlion Nest to a small rebel hideout. On the way he can regain health lost from Antlion Attacks by crushing Antlion Grubs; Each grub releases a pellet that restores 1 health. Later, at the rebel hideout,
Sheckley: The Vorts say as long as we don't step on their grubs, the antlions won't hear us this far from the nest
Nice Job Fixing It, Villain: The confiscation field in the citadel destroys all your weapons... Except the gravity gun, which is made so powerful that it can vaporize a Combine Elite in less than 3 seconds and pick up/throw energy orbs, which are capable of doing even more damage.
Averted. When using the crossbow at long range, the player must compensate for arrow drop.
Also averted with the Gravity Gun. Most evident at the end of Episode Two, when it's used to destroy the attacking Striders.
No Fair Cheating: Early on in the game, when you are walking through the square without any weapons, a Strider walks past in the distance, behind a roadblock. If you put the cheat codes in and blast the Strider with the rocket launcher (which you don't normally have at this point in the game) as it passes, it pauses and glares at you for a moment before continuing on its way.
Letting Alyx or Barney die in any game where they are in combat (which probably will never happen to you anyway).
Losing your vehicle in any of the games (especially weird after you aren't being specifically controlled by a status-reporting G-Man anymore).
If you fail in Episode Two, you get a Vortigaunt-styled message, reflecting the fact that the G-Man is no longer controlling you. ("the Magnusson's misgivings about the Freeman were completely justified.")
No Sex Allowed: Noted early on in Half-Life 2 when Breen reads a letter from a concerned citizen about the "Suppression Field" that keeps people from reproducing. Amusingly, later in the game you come across another "Suppression Device": A giant laser cannon. That'll kill the mood alright.
Resistance Member: When this is all over, I'm gonna mate.
Also, after the Citadel has fallen:
Alyx: Uh, is Dr. Kleiner really telling everyone to... get busy?
No Transhumanism Allowed: Despite the fact the technology is available, and the situation desperate enough that cybernetic modification is rather plausible, only the Combine's human loyalists (who are even called the Transhuman Forces) have cybernetic upgrades of any kind... well, them and the Stalkers — dissident (or just unlucky) humans mutilated into mindless cyborg drones. Even the Combine's "leader of humanity" hasn't upgraded himself, despite talking of his own free will about how the Combine are here to elevate humanity to the next stage. In fact, when the Advisor suggests uploading him into a host body, his initial response is "You must be joking!" Originally, Breen's place was taken by 'the Consul', who rendered himself immortal using the Combine's life support technology.
It might have something to do with the fact that most Transhuman technology is currently under the use of the combine and they take out the "Human" part of Trans Human.
This is half-explained by the fact that City 17 is the largest of a number of "City #" megacities around the globe, and that most of the human population (minus the Resistance, of course) have been ferried around. It would certainly explain how all the Black Mesa folks ended up there.
Father Grigori seems to be one of the few (if not the only) exceptions, who looks and talks like someone raised around the general vicinity of City 17.
Our Zombies Are Different: Zombies are actually living (if gangrenous and mutated) people that have been hijacked by headcrabs. Half-Life 2 introduces Fast and Poison varieties, and Episode 1 depicts what happens when Combine troopers get zombified.
Oxygen Meter: It's also grouped together with the flashlight and the sprint meter in Half-Life 2 and Episode One, which makes underwater sequences harder if you didn't have time to recharge.
P.O.V. Boy, Poster Girl: Gordon Freeman and Alyx Vance. Though Gordon is the player character, the two get equal billing on promotional material, including boxart, and it's Alyx's introduction that really kicks off the plot. It's implied that Alyx was the Resistance's top operative until Gordon shows up.
Respawning Enemies: Antlions will respawn forever from the ground or holes in the pavement; in Episode One you are able to plug up the holes by shoving cars onto them with the gravity gun. One of the NPCs even hangs a lampshade on this: "It's spawning season for the antlions!"
Ridiculously Difficult Route: You end up having to go through Ravenholm (a zombie infested town on a route that "no-one uses anymore") because the other route you were originally going to take gets cut off by the Combine attack.
Near the end of the "Water Hazard" level, as the sunset paints the hills gold.
Ravenholm subverts this with much Scenery Gorn as you travel around the zombie infested town.
The top-down view of City 17 (which is actually a sky view of the western end of New York City) as you're making your way to the top of the Citadel near the end of the game.
Anytime you can see the devastated Citadel in the distance; particularly noteworthy is the start of the game right when you wake up out of the rubble and the first thing you see after Dog digs you out is a down-top view of the burning Citadel◊.
The Citadel core◊ is another noteworthy scenery that just has that tranquil feeling as you move about the area.
The Antlion nest◊; never has an insect hive felt more realistic in gaming then what you witness in this portion of the game.
Half-Life 2 challenges you to beat Ravenholm with only the Gravity Gun.
In Episode One, there is an achievement called "The One Free Bullet." You get one shot to shoot off a padlock and that's the only round you can fire in the game. Feel free to use as many explosives and physics objects as you like, though.
Shooting down the helicopter in Episode Two without missing once is also an Achievement.
Lets not forget "Little Rocket Man" (carry the gnome you discover at the start of episode two all the way through the game and place him in the rocket, inside a special compartment at the end of the game).
Not stepping on the sand in the Half-Life 2 chapter Sandtraps.
Saving all citizens in the last chapter of Episode One. Compared to the rest of the challenges, however, this one is fairly easy.
Keeping all buildings intact during the strider battle in Episode Two.
Shipper on Deck: Eli (and Barney) for Gordon and Alyx. For example, Eli remarking to them that he wants grandkids after the suppressor field is down.
She lampshades it, too; "We came here to get medical supplies; I got a shotgun!"
Shout-Out: Lamarr the Headcrab — Hedy Lamarr (get it?) was a well-known, glamorous actress between about 1940 and 1960 (and, incidentally, also co-invented an early form of spread-spectrum wireless communications), but she's probably better known for suing Mel Brooks for naming the main villain of Blazing Saddles "Hedley Lamarr." So what if we name a de-fanged brain-devouring parasite after her...
Hedy Lamarr is probably known even better for being one of the first women ever that appeared fully naked in a movie (Extase, a Czechoslovak film directed by Gustav Machaty in 1932). Naming a pet headcrab after her can be viewed as way more disturbing than the previous reason.
Also, in Episode Two there's a Shout-Out to the original Half-Life by Doctor Magnusson. One of the things you can do in the original game before putting on your HEV suit and starting the experiment is visiting the employee break room, which contains a microwave oven. Repeatedly pushing the buttons on it (which many players did due to the satisfying 'beep' it makes) will eventually make the microwave casserole explode and cover the interior of the microwave as another scientist asks Gordon what he's doing. Over twenty years later, Doctor Magnusson still holds a grudge over that casserole..
Dr. Breen's name may or may not be a reference to Ćon Flux.
The Combine are actually based on the hallucinations of Chief Bromden from One Flew Over The Cuckoo's Nest. The monstrous, imperialistic, mind-controlling alien cyborgs he describes in the book are even called The Combine.
An obscure visual shout-out: Some of the Resistance graffiti shows oranges and lemons. This is a reference to the British childrens' poem "Oranges and Lemons", as it was used in the book 1984.
Skeleton Key: Alyx appears to have an electronic one about half-way through Half-Life 2, which opens all doors in Nova Prospekt and can also hack Combine computer consoles.
Her EMP hacking device is used in the eleventh-hour heel face turn by Judith Mossman and figures heavily in Alyx's contribution to re-invading and escaping a second time from the Citadel and City 17 in Episode One.
Shut Up, Hannibal!: Sort of: you can use the Blue Gravity Gun to either rip out or just blast the Breencasts. Except the last one, which Breen does on an unbreakable screen.
Sound Effect Bleep: When you first break into the Citadel, Barney tells you, "And if you see Dr. Breen, tell him I said 'fuSLAMou'!" He is cut off by the sound of the wall segment that D0G was holding up crashing back down. The voice file itself isn't bleeped, however.
Stealth Pun: Lamarr the headcrab, which is just obscure enough a reference that it can be missed. Explained in a blink-and-you-miss-it comment by Kleiner after you leave his lab for the second time that her first name is actually Hedy.
Not many people realise that Rasputin was known as Father Grigori - given that Half-Life 2's Father Grigori just won't die, he might actually be Rasputin.
Sterility Plague: The combine have set up some sort of device which makes humans unable to reproduce. In the expansions this has been deactivated, leading to Dr. Kleiner to suggest that repopulation can begin.
Alyx Vance: Uh, is Dr. Kleiner really telling everyone to... get busy?
Suspicious Videogame Generosity: Your protective suit has 100 points of armour when fully charged, and the wall chargers can give you 75 points (on easy). When you make it into the Citadel at the end of the game, the wall charger charges your armor up to 200 points, and your health to 100 points. Uh-oh.
Presumably what happened to the cat Dr. Kleiner used to test the teleporter. Alyx hasn't heard about it until Barney mentions it in-game.
Barney: Is [the teleporter] working? For real this time? Because I still have nightmares about that cat.
Dr. Kleiner: Now, now, there's nothing to be worried about.
Alyx: What cat?
Dr. Kleiner: We've made major strides since then. Major strides.
Alyx: What cat?
After hooking back up with Barney during the rebellion, he has a flashback about it. Whatever happened, it must have been traumatic, as he was plainly traumatized.
Doing this to Kleiner's mini-teleporter is an achievement named "What cat?".
Ten-Second Flashlight: Played straight, where it shared the same energy meter as sprinting and holding your breath. In the original game the flashlight had its own meter, and it was given one again in Episode Two. In the latter case, the light recharges pretty quickly, but it takes ages for them to run out.
Lampshaded too, as Alyx may give a snarky remark that you really need to get Dr. Kleiner to give you a new battery for it.
Tentative Light: The flares in Episode One will do this before going out. In Episode Two, your flashlight will blink before turning off; it's much easier to get it back on than before, though.
The Dev Team Thinksof Everything: In Half-Life 2 episode 2, if the player manages to get through a small crevice past the antlion guard, there is floating text that reads "How did you get here?" further into the cave.
After obtaining the gravity gun (or zero-point energy field manipulator), if you pull the barrels down before Alex says to try pulling them down, she skips the mention of the barrels and instead says "Try pulling something towards you."
There Is No Kill Like Overkill: Hinted at by Dr. Breen in the Combine Teleporter. "You, on the other hand, will be destroyed in every way it is possible to be destroyed, and in some ways which are essentially impossible!"
The couple from Half-Life 2 have shown up twice in the game (once before the apartment raid, the second after the uprising), and once in both episodes.
Training Dummy: The disabled Strider at the end of Episode Two, as well as a dead Combine soldier tied to a post after you get the pheropods in Half-Life 2.
Triang Relations: Alyx Vance, Eli Vance and Judith Mossman are an example of Type 7a. Alyx loves her father Eli, but does not get on with Judith. Eli and Judith, however, seem to feel something more than professional respect for each other.
Trigger Phrase: Not exactly a phrase, but the silhouette of the G-Man appearing on a monitor causes Alyx to go into a trance and deliver a message to her father. She has no memory of this. Makes one wonder how much control the G-Man has over her.
Trojan Prisoner: Employed in the beginning. Barney, as an undercover Civil Protection officer, takes Gordon in for "interrogation" and helps him escape.
Uncanny Valley: Averted in Half-Life 2, thanks to some damn fine graphics, motion capture, and animation - except deliberately invoked in the G-Man's case. The subtle asymmetries of his face, some odd mannerisms, the ways he speaks, as though he knows how to make the sounds of speech but isn't sure how they all fit together... all help make him a very creepy supernatural stalker.
Underground Railroad: A network of safehouses that guide citizens hoping to escape from City 17. When the Combine bring the hammer down trying to stop Gordon Freeman from escaping, much of the railroad ends up being destroyed helping him get away.
G-Man: Tell your father [Eli Vance] to prepare for Unforseen Consequences.
Which is the name of the level immediately after Gordon and Eli caused the Resonance Cascade and started first the Xen and then the Combine invasion of Earth. It was not an accident which he exploited to have the Nihilanth killed by Gordon, he had planned and arranged for everything from the very beginning.
Gordon's stasis apparently seemed like a day to him, but two decades pass.
Also, when Alyx and Gordon go through the teleporter near the end of Half-Life 2, a week passes in what is to them a blink of the eye.
Gordon's rescue from going back into stasis and Alyx's save from being Ground Zero Girl at the detonation of the Combine Teleporter were both accomplished outside of normal timeflow through the efforts of friendly Vortigaunts. The G-Man was not amused by this development.