I prefer mostly realism, but it has to be tempered with fantasy. In real life a man who takes a bullet or shrapnel is most likely out for weeks or months, if not dead or crippled. There's a reason they say "war is hell" and not "war is challenging and fun." That being said, I HATE random power-ups, although I don't have a problem with med-kits (although I do prefer them to require a deliberate action and a bit of time.) Go figure.
I prefer a realistic world and character models, as opposed to anime or cartoon/cell shaded style, in all my games. Counterstrike I love; Team Fortress leaves me cold. And I prefer accuracy to reflect the real world, including trajectory. Within those constraints, I like near-future sci-fi like Crysis, or low fantasy; I generally dislike far-future sci-fi such as Halo (although I played the first one and it was decent) and high fantasy. But very far-future sci-fi or high fantasy can be fun as long as it is well done and internally consistent.
As mshcherbatskaya said, it's most important that a game be realistic within its own defined parameters. If I have a suit that provides armor and stealth and regenerates itself, fine; I can deal with that. If I have nanites that slowly repair any damage I take, whatever. If on the other hand I come to a two-rail fence I cannot cross, or a door I cannot pass through (exclusive of force fields and steel security doors and the like), now I'm pissed. And I too demand good AI to rate a game highly; I would much prefer fair graphics and good AI to great graphics and poor AI. And even a game with very little reality or internal consistency (say, Psychonauts) can be fun if it's well done, has believable characters with good AI, and presents a well thought out rationale for its lack of reality or internal consistency.