When I first played any Bethesda game I used F5. Now, I do not use it anymore because of hitches, CTD, bloat, etc.

I'm finding that it can cause crashes in mass Effect 1. I've conditioned myself to not press it every 30 seconds now.

On my old PC, New Vegas would freeze everytime I hit F5

That's strange because New Vegas is actually the one game I have that has absolutely no problems with my incessant clicking.

If theres lava, I jump into it to see if theres any cool burn to death animations.

I always catch missingno.

I always stay away from the water.

After so many giant sea beasts trying to gulp you if you get to close, well you tend to be cautious. And whenever there's a lake to explore i always search the the bottom for hidden treasure.

Contradiction?

After I've made sure its ok, I jump in(if able too).

I always feel bad about killing a cool boss in a game, for the reason of it being too easy. I'll usually do what some of you have already mentioned (save all the good items until the boss fight, only to refuse to use them anyways) and then I'll try to pace myself, so as not to kill the boss too fast. I save it before each boss just so that I can fight it again if I found it to be too easy, or if it looks too cool. I'll usually place restrictions on myself if I felt the need to fight a boss again.

It was weak parts, so that was why *cough* celeron *cough*

--If the game allows it, I save obsessively. Before every door, before every dungeon, before every room, etc. Often they're new saves, not over-writes, so I end up with crazy numbers of save files. It's less about paranoia about crashes as it is "what if I want to go back and repeatedly replay that little scenario/area for some reason?" (tests, idle lol's, for videos, etc)
--Explore every nook & cranny because I must know what's there.
--Too much time spent looting/hauling loot and shopping for items/materials (the first time or two) because I must know what's possible and I must be rich enough to buy what's possible.
--Similar/same as povuholo - I save a lot of items forever, even low-end stuff, that I end up never using, just because I might need it or at least might want to experiment with it someday. It's especially bad in games where items can spawn with so many different modifiers/combos, that while you may find an item with similar stats, you may never find the exact same one again. It's like a "Digital Hoarding: Buried Alive" level.

Can't think of any habitual gameplay idiosyncrasies that carry over from game to game (like always having a teddy bear in my backpack).

If i have full health, full ammo, top level weapons and armour, i know its time to die. Also i never take my armour off, i know the time i put my civvies on, 5000 ninjas spawn on top of me slice me up and disappear.

I've been playing Bioshock and I find myself crouching when I enter a room that might have enemies in it, in the hopes that I can get a sneak attack on them. I'm not even sure if crouching lets you sneak, but I do it anyway.