Half-Life 2/YMMV

Anticlimax Boss: It really depends on how much ammo you have, but if a couple glitches are exploited and the right weapons are used, it is possible to easily kill the Nihilanth in three or four minutes. The Gluon Gun in particular is capable of absolutely destroying him.
Awesome Music: This.
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!")
Paranoia Fuel: When the G-Man is no longer subdued.

G-Man: Doooooctor Freemaaaaaaaaan...

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.
They Wasted A Perfectly Good Game Mechanic: At various points in the game, you can play as The Beast Master (Antlions), The Turret Master and (via Squad Controls) The Minion Master. Not only that, but they all use separate control schemes. They could have had a late level feature a huge Hold the Line mission using all three at once.... but nope.
- Then again, considering all the Dronejam going on with the Antlions and rebels, it's probably for the best that they were never combined.
Ugly Cute: The Antlions after you get the Bugbait, who follow you like eager puppies and attack anyone you tell them to. Before then, of course, they were pure Nightmare Fuel.
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.