These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Best Level Ever: In the Citadel, once your gravity gun gets upgraded. At first it serves as an Oh Crap moment as almost all your weapons get taken away. The gravity gun, which up until now has been unable to directly damage anything larger than a headcrab, is now your only weapon. Then you find out that it's been powered up, and can now kill human-sized enemies in a single hit. You can rip computer consoles off the wall, and the environment provides you with an unlimited supply of energy balls, which were previously Too Awesome to Use. Then you find out that the wall chargers, which at best would fill your suit power to 75%, now not only fill you all the way, but boost your suit power to 200% and heal you as well. You are now an unstoppable force, able to take out enemy soldiers effortlessly, and deal with former boss enemies with almost no trouble at all.
Broken Base: The We Don't Go To Ravenholm... level. It's either considered the Best Level Ever or That One Level for players, because it goes away from the traditional first person shooting throughout this level; having to rely mostly on the world traps and idle objects thrown by the gravity gun due to lack of gun ammunition. There's other debates regarding the level being if it's out of place for getting in the way of the war against the Combine or not and if the mission is too scary for some players having to fight through a town infested by zombies at night.
If you know your Greek Mythology, it's fairly obvious that the Vortigaunts' portrayal in this game is based on the classic portrayal of the Cyclops. Outside The Odyssey, most classical Greek writers like Hesiod described Cyclops as wise master craftsmen with control over electricity (they were said to be the crafters of Zeus' thunderbolts), in addition to being one-eyed giants.
Iron Woobie: Alyx Vance. For the first years of her life, she lives in the dormitories of Black Mesa, a dangerous and accident-prone research facility. Then, her mother is killed during the Resonance Cascade incident. She only survives because the G-Man saves her life, putting her in a debt that she repays in Episode Two. Fast forward to when she meets Gordon Freeman. She is then separated from him by a malfunctioning teleporter, forcing Gordon to get to Eli's compound himself, fighting Combine along the way. Her father is then captured and the rest of the game is mostly spent trying to rescue him. In the end, when the dark matter reactor explodes, killing Breen, she only survives because the Vortigaunts teleport her out of the wreckage. In Episode One, the evacuatory train Gordon, Alyx and the gang have worked so hard to get running crashes in a wooded area because of a portal storm. In the beginning of Episode Two, she is stabbed twice by the long blades of a Hunter and survives, once again, because of the Vortigaunts. Finally, after Eli, Magnusson, and Kleiner launch the rocket and Gordon and Alyx are about to get into a helicopter to save their friend Mossman, a pair of Advisors smash through the building and kill Eli (her father) right in front of her.
Memetic Mutation: The headcrab zombies' painful cries are actually reversed audio of a man screaming things to the effect of " Dear God, help! HELP ME!!," but it's also easy to interpret them as a straightforward Mondegreen: "YABBA! My icing!" (Or "My eyes sting!")
Most Wonderful Sound: The high, two-tone note that a Metrocop's radio will emit after they die. When a bunch of them play together after a particularly bloody fight, it's like the music of the angels.
Narm: The high pitched screams of the headcrab zombies fall into this, particularly if you're familiar with some of the more popularmondegreens.
Porting Disaster: It's no one's fault, really (the Xbox, while being the most powerful console of its generation, was not fit to run something as meaty as the Source engine), but the quality of the Xbox version is simply too far away from the PC original to make it worth playing.
The Playstation 3 version of The Orange Box was pretty shabby too, with Valve (at the time) not having that much faith in the PS3 and it being outsourced to another company entirely.