Awesome but Impractical: The Chainsaw. It can tear bugs apart... but so can the standard melee attacks, and those are less likely to get you killed or wounded. However, unlike the standard melee attack, you can move freely while using it. A good chainsaw user can simply run into a mob of aliens and leave chunky bits in his wake, while anyone attempting to punch his way through the mob would simply get mauled between punches.
Also, the chainsaw is often used by speedrunners, as it can be used to both remove the boulders in the last mission without using the slower mining laser and break down doors faster than opening them by hacking.
There are two hidden items in the game: The Assault Jets and The Blink Pack. Each can only be found on a specific stage. The first is your only jumping option in the game, but has very few uses and can't do much to enemies. The Blink Pack lets you teleport. It has unlimited uses, but a lengthy recharge time. It will also zap any enemies around both ends of the teleport. However, you're limited to teleporting to places on your screen, and only places you could reach on foot. Both take up a slot that could be much better utilized.
Scratch the impractical part, advanced players can combine both to abuse gaps between map props and easily solo any map where those items are present on the highest difficulties.
The Smart Bomb is certainly an effective weapon if you're being overwhelmed by aliens, but it's single-use and ends up being tremendous overkill in most situations. Most of the time you can get the same effect just by dropping a freeze grenade at your feet.
The Grenade Launcher. Big splash range when things get crowded but causes extremely high friendly fire damage plus the shots arc instead of flying straight.
It becomes very practical on insane difficulty when there are large aliens on every map and you need to kill them as quickly as possible.
Better to Die Than Be Killed: Without healing, a parasite will kill you, and spawn another four that pose a significant risk to your teammates. Better to die in battle, charging into the largest mass of aliens before dropping all your grenades at your feet.
Boxed Crook: Wolfe is an ex-con who chose military service over life imprisonment. In a less-seen variation, he turns out to enjoy the military far more than his previous life of crime, and sticks with it as a career.
Bullet Time: The adrenaline rush. Happens automatically at some points in the game, but there is also an item that grants it. In both cases, it affects the entire squad.
Also, whenever a character dies, we have a slow-mo cutscene of him or her dying. Generally, this is used to get the last edifying reactions from their squadmates. These are normally variations on theme of "oh shit", "noooo", "lol" or "oops".
Chainsaw Good: Oddly enough, the blade of this game's chainsaw is horizontally-aligned. Still chops the Swarm up good though.
Probably because a vertical chainsaw would be harder to maneuver side-to-side through multiple swarm while being held comfortably.
Character Level: EXP is awarded at the end of various stages. To add to that, items and weapons unlock when you level up; thus, the more you play, the larger your arsenal.
There is also the Rank system, where you can trade 26 levels for another Rank. This has no positive impact on the game however, and actually takes away your weapons if you choose to do so (level 27 being coincidentally the highest you can get).
Dummied Out: A look in the sounds folder reveals several Marine voice files that address a tech named Flynn, who is not present in the game.
Alas, poor Flynn. His fate is unknown - he was present in the Unreal Tournament 2004 mod, but was replaced by Vegas for reasons unknown. Could have been voice actor troubles.
There also exists a fully working Queen alien, complete with massive "boss" health bar on top of the screen, but she doesn't appear anywhere in the game's original campaign. You can still place the entity in maps or spawn it from the console.
Game Mod: Like Killing Floor, it started out as a mod for Unreal Tournament 2004. Valve hired three of the original developers, who then remade it on the Source engine in their "spare time". Like other games on Source, it also supports custom maps.
Plus the mods for the game itself that the community creates. The game is also fully open-source, meaning you can edit the files however you want.
Gatling Good: The Special Weapons class starts with an Autogun, which is like a less powerful minigun that can lock onto targets. You can unlock an actual minigun when you reach level 17, which trades off accuracy for a faster fire rate.
Goomba Stomp: If you land on an enemy after jumping with the Assault Jets, they go squish.
Guns Akimbo: Dual pistols and Personal Defense Weapons are available. The pistols' description notes that they are issued in pairs.
Harder Than Hard: Insane difficulty. The game on Normal difficulty is challenging enough, this makes the swarms faster, stronger and spawns them in more places (you get parasites on the first level). It demands good teamwork otherwise you won't get very far.
An update outclasses Insane with Brutal difficulty for people who say Insane isn't hard enough! How hard is it? Hard enough that none of the developers could complete a mission on Brutal difficulty!
Onslaught mode, another new feature added to the game, can be borderline Nintendo Hard since the aliens now are randomly generated and a huge swarm may randomly appear to slaughter the team, just like in Left 4 Dead. As if this and Brutal difficulty wasn't enough, Valve also added a Hardcore Friendly Fire option that makes all friendly fire deal full damage to teammates and people who are set on fire stay lit a lot longer. And yes, this can turned on for Insane and Brutal difficulties.
There's an achievement for beating a mission on Brutal with Onslaught and Hardcore FF turned on. Have fun!
Inventory Management Puzzle: You can only carry one of the game's many equippable items at a time. It's your choice whether to take something that benefits you (Health Kit, armor upgrade) something that harms the Swarm (tripmines, rocket pack) or something that helps the team (Damage amp beacon, flares, Welder).
Killed Off for Real: Unlike Left 4 Dead and many other multiplayer shooters, players who are killed stay dead until the next round.
Laser Sight: All weapons in the game have one equipped. Pretty much necessary, given the top-down camera. The deployable tripmines also use these.
Leeroy Jenkins: Players who attempt to speed run a map and not tell anyone else (which will usually get the Leeroy killed or their team killed) and/or guys who mishandle power weapons like the Flamethrower and constantly cause friendly fire damage with them.
Mercy Kill: If there are no medics, healing packs, or shock armor left, an infected player has no chance of survival. Better to burn them to death than wait for a swarm of parasites to emerge from their corpse once they die.
Mook Maker: The last level features a giant spider-like enemy which spawns tiny suicide bombers, and releases a whole pile of them on death.
If a parasite kills a player, it also releases a bunch upon death. Protip: Don't stand near a player who will be killed by a parasite. And don't stand near other players if you're going to die to one!
More Dakka: The Autogun really lays down some covering fire, but the Minigun does even more, at the cost of accuracy.
X33 Damage Amplifier item does exactly what it says for characters standing in it's radius, doubling damage done for the duration.
Including damage to teammates. As if shotguns weren't dangerous enough...
Noodle Incident: Several of the marines mention previous battles where things got pretty rough, to which Sarge replies that he hopes their current mission doesn't get that bad.
One Bullet Clips: Averted. When you reload, you lose the remaining bullets of the previous magazine.
One-Hit Kill: The officer class has access to napalm mines that can kill even Shieldbugs on contact.
Getting flattened by a falling door will do this to you.
One-Hit Polykill: The Special Weapons skill Piercing Bullets gives each shot fired a chance to pass through multiple enemies in its path.
Also see the Railgun and Scoped Rifle.
One-Man Army: Averted. You can't start a campaign without at least two people. Unless, of course, you enter a console command. You'll probably die if you go it alone, though, even on easy.
Palette Swap: The Tesla Cannon and the Mining Laser both share the same models with different colors.
Power Armor: What the characters wear, color-coded based on their class. Officer = Green, Medic = White with red stripe, Heavy Weapons = Bluish grey, Tech = Yellow. There are also some design differences; medics, officers, and techs have different flavors of backpack for their armor while the heavy weapons character has what looks to be an ammo drum.
Recursive Ammo: Jaeger and the Hand Grenade Unlockable. Normally, the Hand Grenades are basically a copy of the standard Assault Rifle's Grenades. It's worth noting, however, that the Explosives skill increases the damage by 10 per point, and adds one cluster munition to the main grenade per point. Jaeger has Explosives 4. That's five explosions from one grenade dealing 120 damage each.
Take a Third Option: Doors either require a key card to unlock or need a tool to unseal it if it was welded shut. Other doors require a Tech to hack it open. Sometimes, you can find a way around the doors, but if the first two options won't work, you can just take a chainsaw and rip the door down. Just be careful with where the door falls down!
Theres No Kill Like Overkill: The Smart Bomb tends to kill every single thing on the user's screen.... and still have some left over missiles flying around.
Token Minority: Vegas is the only black person in the Valve version of the game. On the other hand, it has two women, Wildcat and Faith.
Videogame Flamethrowers Suck: Averted. Now that the Tesla Cannon is no longer the undisputed King of Weapons, the Flamethrower has mostly taken its place. Why? A single puff of fire ignites an enemy (only prolonged fire ignites teammates, thankfully... but when it does, it isn't pretty), making it highly ammo efficient. Each puff is weak, but the burn damage is high; a single flamer sweep can turn a horde of drones into a staggering, flaming mess: no squad members injured. Only an idiot goes into a nest of Parasites without one. Oh, and its alt fire extinguishes fire... how's that for handy?
Weaponized Exhaust: The two hidden items, the Blink Pack and Assault Jets. The first is a teleporter which has a side-effect of electrocuting everything around you at start and finish. The other is your standard rocket jump boots, and can light enemies on fire if you're next to them when you take off.
We Cannot Go On Without You: Any level that requires a Tech marine will become an instant failure if they are killed. Sometimes it's a good idea to have two Tech guys as a backup plan.
Weld The Lock: If you have the Welder equipped, you can weld doors to temporarily hold off the Swarm. You also find some doors already welded that if unwelded, may reveal extra items if not the way forward.