In RTS games against the CPU, I only ever attack with a very superior force.
In RTS games on the internet, I religiously follow Sun Tzu's Art of War. "I'm a bit bigger than them. Part of my forces pincer, the rest attack head on", "I'm twice as big. All head on!", "We're equal. I will avoid them", etc.
In RTS games, I tend to build all my passive-gain resource buildings (power plants, farms, etc) in neat lines at the very back of my base.
RTS games again. If a plan goes off smoothly, I laugh like a villain. "Muhahahahahaaa!"
FPS games. I reload constantly. Fucking constantly. Kill an enemy? RELOAD! Fire a few bullets at someone who ran off? RELOAD! I hate not having a full clip, and FPS games that won't let me do the "magic reload of not losing bullets from it" thing annoy me.
FPS games. Absolute refusal to use sniper rifles unless it's single player. This is a "do unto others" thing, really, because I consider anyone who camps in multiplayer games a cowardly jerk who's too stupid to actually play the game and prefers to just stay in one place and get easy kills because the game's level designers were fucking morons. This does NOT count if it's really justified (IE: Capture the Flag or mission-based levels when you defend your objective). Only in DM or TDM. But even if I play CTF, I won't use sniper rifles. So it counts as a strange habit.
3rd person games: If it's not slower than just plain running, I will jump, roll, whatever I can, aside running normally. No reason whatsoever, really. If it DOES make it faster, at least I feel good. (Like LoZ:OoT and side-hopping) Mostly I do it for no reason though.
Horror Games: Extremely constant reloading. Makes my constant FPS reloading look silly. Yes, this even goes for early Resident Evils where you only ever reloaded in the menu. I kill a zombie? Menu reload! Even worse since I could (and would) do it when there were multiple zombies around, still walking.
My complete hatred for having any injury whatsoever counts too. In an RE game, I've been known to use First Aid Sprays when my HP reaches yellow. (Even worse in RE4, where I'd use green herbs if I even got a tiny nick from a ganados)