This is a fantastic thread, I just had to post some of mine.
1) I too have been known to occasionally game nude. Well, oay, I'm not KNOWN to game nude but I'm admitting it here and now.
2) I too, am also a reload junky, which, when using the breen gun in COD4 ensures many a failed deathmatch.
3) I also like to nod my character's head in FPSs, just for the immersion factor.
4)In a slightly twisted fashion, I tend to cackle maniacally when I perform a particular gruesome or gratuitous kill in a FPS or other typically violent game. My favourite in particular was when I turned a corner in Half Life 2, didn't even aim my gun, and nailed a combine soldier with one magnum shot to the head. The bloodstain hit the wall at a perfectly straight projectory, he slumped over the table he was behind, and despite having his gun primed over the table he was hiding behind, he barely had a chance to fire a shot.
5) If I'm playing a game with catchy music, I tend to narrate my onscreen actions in time and pitch to the melody. Playing Symphony of the Night and singing to a skeleton worrior about how exactly I'm going to kill him is both juvenile and entertaining.
6) If I've been left alone all day with nobody to talk to, 9 times out of 10 I'll fire up my console and if I'm not talking to my onscreen assailants, I'm talking to myself, congratulating myself for good kills or puzzle-solving skills.
7) I get through some of the hardest puzzles in a game, then get stuck on the really easy ones, give up within a half hour, look at a walkthrough, and then feel really guilty for the remainder of the game. I did this earlier today for the Milkman stage in Psychonauts. The solution was so simple, and I love the game so much that I'm still feeling dirty for it.
8) If a game is bright, colourful, relaxing, strange, and simple (Katamari Damacy being a great example) I tend to wait until I'm really stoned before playing it. Makes it so much more fun. Ditto with scary games - I don't normally get freaked out by horror titles, but FEAR, a few tokes, and surround sound, and you're in for a terrifying treat.
9) As a kid, I used to play games to enjoy and complete them. Nowadays, I want to see a game through to the end to critique it. I do still enjoy playing through my games (why else would I play them?) but I aspire to be either a game designer or critic, and when I play through a game it's very rarely for storyline closure, but so that I can look back and analyse what did or didn't make the game great.
10) I absolutely detest Oblivion. It's universally loved by many, others are impartial, but I find every last piece of binary that holds that game together totally objectionable.