Quake 2 did have a militaristic, semi-realistic feel to it's graphics and story. Gray, Green, Blue, and Brown were the predominant colors in a radiosity-lit world, and it's weapons and level designs were almost functional in appearance. The interface had a dog-tagged look to it, and even the manual had the worn subtleties of a militray fieldbook. No loud contrasting colors, no deeply etched vertex shadows, no sci-fi party house paraphernalia sticking out of walls for no reason (Think Unreal 2.) The intro has echos of Normandy in it, with countless infantry futilely bombarding fixed fortifications with the goal of destroying a powerful defense mechanism (The Big Gun.)
However the context (Especially in the enemy character designs) and gameplay in which all these realistic elements were placed in was completely sci-fi fantasy. and therein lies what IMO was one of it's best jabs at immersion. It feels like an action game but it's realistic look added a suspension of disbelief.
"Nothing livens up a robotic hymn of doom more than an amazing pair of jugs." - Brak