Quake 4/YMMV

Complete Monster: The Strogg race as a whole.
Crowning Moment of Awesome: The pose that Kane strikes during the ending, upon destroying the Nexus. Utterly Badass.
Crowning Moment of Funny: After Kane's rescue:

Sledge: Welcome back, Corporal Kane. May I be the first to say you look like shit.

Demonic Spiders: Berserkers, Grunts, (fast, Made of Iron, like to berserk and Deadly Lunge) Heavy Hovertanks and Light Tanks.
Fridge Logic: Aside from aesthetic reasons, Quake II takes place only shortly prior to Quake IV; however, practically everything has undergone a redesign (returning enemy types look different, architectural styles are dissimilar, etc). However, bear in mind that, in the chance of a Quake II remake with the IdTech4 engine, the game would look pretty much similar to Quake IV. Plus, Quake II was done by Id, while Quake IV was made by Raven.
- Anderson's backstory says that he isn't afraid of using weapons against the enemy. Now, why don't he just shoot the Scientist?
Goddamned Bats: Certain flying enemies.
Nightmare Fuel:
- The "Stro ggi fi ca ti on" scene. Kane's only reaction to the whole thing is remaining completely silent and only momentarily emitting the same grunting sound, that sounds more like someone getting beat with a stick than someone being brutally mutilated. It might push it into Nightmare Retardant territory.
- The Strogg also like using dismembered human corpses (or dismembered living people) as decorations, as areas in this game and it's canonical predeccesor shows. It Gets Worse: they use them as batteries.
That One Boss: Voss's Stroggified self, who's just about as deadly as the Makron. Made of Iron, fast for his size, armed with Rocket Launchers and a Dark Matter Gun, can summon Mooks, and recharges his health up to three times. It takes a lot to bring him down, even at Training Level! And what's more, it doesn't help that he's a Tragic Monster.