In the Mod Sven Co-op the developers added a new Xen Enemy called Tor. He can summon Alien Grunts that distract you as he shoots an energy that lifts you up and let fall damage do its job for him. Think you can just zerg rush him? Too bad he can launch and shockwave you guys away.
Basically any Strong enemies in the mod got a buff in 4.6, Gonomes, Alien Grunts, and Human Grunts can run and fire as well as melee you.
People really like Adrian Shephard, to the point where Valve may bring him back, as most fans seem to believe he's always behind Gordon.
Also, DOG. Valve responded to his popularity by giving him a major role in the intro of Episode One.
Barney, to an extent. People have shown to be really concerned about where he is in Episode Two.
And, of course, the G-Man himself has gained quite a bit of popularity, especially through Garry's Mod.
The All-Knowing Vortigaunt and the Nihilanth, simply because people love speculating about what they're telling you.
Genius Bonus: The series is full of references to scientific concepts (starting with the game titles), particularly quantum mechanics and cosmology. This game is where most people first heard about The Challenger Deep, Dark Energy, Singularities or The Calabi-Yau Model.
The teleportation theme seems to be a bit better researched than in most games, as well. It appears to be based on a wormhole principle, which, naturally, requires that Dark Energy everyone talks about.
The headcrabs. Especially the poison ones, which are specifically designed to screw with your ability to deal with other enemies.
Manhacks apply, as well. Those buggers just swarm all over you. Plus they fly! Once you have the Gravity Gun, though, they lose a lot of their annoyance to the "grab and punt" kill method.
The Antlions, who often push you onto sand, causing more of them to come!
Doubles as Fridge Brilliance. Think about bees. A single antlion spotting a threat attempts to force said threat to cause more to come to its aid in case it is more than it can handle on its own.
Hilarious in Hindsight: Half-Life and Lost were made by fans of each other and have referenced each other. Then, in 2017, Marc Laidlaw's plot outline of Episode 3 reveals that the plot had involved the ship the Borealis constantly traveling through time and the only way to find it is to predict where it's going to be, the exact same method given by Lost on how to find the Island that was that show's focus.
Hypocritical Fandom: Gordon Freeman never speaks considering that creator Gabe Newell wanted to give Freeman a greater sense of embodiment, and quite a few people like him because of that. Unfortunately, some of these same fans tend to complain about other games with mute (or laconic) heroes, even if it's for the same reasons.
Magnificent Bastard: The G-Man is an enigmatic bureaucrat who uses his supernatural abilities to carry out the whims of his "employers". Always watching from afar, the G-Man monitors people who exhibit great survival instincts to employ them under his cause. Responsible for bringing the crystal sample that cause the Black Mesa Incident, the G-Man went against the orders of his superiors to rescue Alyx Vance and Adrian Shephard for his own purposes. The G-Man would rearm a nuclear warhead to destroy the entire facility to cover-up his tracks. When Gordon Freeman killed the Nihilanth, the G-Man recruited him as Xen was now under the control of his employers. Twenty years later, he sent Gordon to City 17 to help overthrow the rule of the Combine. Although he was held back by the powers of the Vortigaunts, the G-Man waited until they were distracted to order Alyx to warn her father about the Borealis. With motives known only to himself, the G-Man removes any sense of choice from the people he manipulates.
Memetic Mutation: What happened when someone noticed that one of the scientists working on the Large Hadron Collider bore a resemblance to Gordon Freeman. The Internet ran with it and went so far as to send a care package of items Gordon would need to save us all once the activation of the collider opened a portal to Xen. The G-Man was spotted too, so you know we're all dead.
Wait a minute, TV Tropes, in total, has eight letters. Two on TV and six on Tropes. Six divided by two is three. HALF-LIFE 3 CONFIRMED! Explanation Thanks to the Development Hell that Half-Life 3 is currently in, many people have gone and went to crazy lengths to "confirm" Half-Life 3 is coming out.
More Popular Spinoff: Counter-Strike and Team Fortress Classic. Originally, they were simply Gaiden Game multiplayer add-ons to Half-Life, being sold at retail mostly in bundles with the Half-Life games, sharing the bulk of their assets with it, and implicitly being set in its universe by the presence of things like Black Mesa logos in the retail maps. For a while, the only way to even get Counter-Strike: Source was to buy a copy of Half-Life 2. However, the massive success of these games' sequels, Counter-Strike: Global Offensive and Team Fortress 2 has arguably made those series more famous and successful than the one that they spawned from. To wit, Team Fortress 2 was at one time the most popular PC shooter period, while CSGO sold 25 million copies prior to going free-to-play, which is more than Half-Life, Half-Life 2, or The Orange Box... combined.
Paranoia Fuel: Headcrabs are everywhere, especially when they leap out at you from vents when you don't have your flashlight on. Compounded when the sequel added poison and fast varieties of headcrab. An unofficial design principle for custom maps runs thus: "A headcrab in every vent is boring; a headcrab in one-fifth of the vents is terrifying."
Adrian Shepard of Opposing Force definitely comes off as this to those wishing for him to finally make a reappearance ever since the G-Man put him in indefinite detainment at the end of the game.
Race X, also from Opposing Force, comes off as this as well to those who thought this alien race was more fun and unique compared to the Xen aliens.
The Nihilanth from the original game. Despite being the Final Boss and being built up to a lot throughout Xen, it's never brought up past throw-away lines by the Vortigaunts after the original game's ending. There's also everything involving the Nihilanth, from why it was trying to take over Earth to what it was running away from. Outside of the game's original ending, everything involving Xen besides the Headcrabs and Vortigaunts are either not mentioned or implied (but never stated). The theory that what the Nihilanth was running from was the Combine is never actually stated.
Ugly Cute: Lamar, Dr. Kleiner's pet headcrab. Every other headcrab is Nightmare Fuel defined and the only physical difference Lamar has from the rest is that she's been debeaked, but she's otherwise a domesticated pet who feeds on watermelons whose startling but completely harmless attempts to latch onto other people's heads and tendency to hide in vents is strangely endearing.