1. I must always obey the traffic laws in any sandbox game. Even in chase scenes I don't want to hit cars not because it makes me get to the objective faster but because I feel bad for the people I hit. Games like GTA4 and SR3 I often times get so rage filled when I just knick an NPC car that I have to reload. I park my cars as perfectly as I can. I even make sure that the tiniest scratch that someone else has caused to my vehicle is always fixed at a shop right away! (unless I reloaded). This even extends to walking where I'll always use sidewalks and paths (unless being chased) and wait for cars to let me cross the road. I try to walk places, too, and don't just run everywhere, unless I'm actually in a hurry.
2. I don't like forcing other NPCs into my business. This can range from making sure I never shoot a bystander, even if I'm supposed to be a psychotic genocide artist (like in SR3) to a super-powered office of the law (Crackdown). This can get to a very disturbing level of dedication in situations like in True Crime when you fire warning shots and a civilian pulls out a gun to defend themselves, I'll attempt to disarm them if possible but always run away (old lady chasing me down with an automatic shotgun...yeah...). This couples itself with #1 where I don't like hitting NPC cars while chasing down bad guys or am being chased and trying to get away.
3. I will always max out a character. RPGs that don't let me actually do this annoy me at a basic level. This even extends to games that lock certain benefits for going down particular paths (like Paragon/Renegade in ME, which I totally cheated on the second I found the conversation wheel glitch). I must make sure that my character is the best at everything. The only exception is weapon styles, but I must be a master of at least one weapon.
4. I'm thematic. I don't go for sword and shield because it gives +15% more defense than two handed swords which gives you only 5% more damage so the stat balance is in favour of it in the end, I go for it because I'm determined to play a protective knight. So what if loin clothes only give 2 armour and I'm almost done the game? I'm a barbarian of the frigid Nordlands come to pillage all your furs! When I get armour, I buy it in sets and make sure it matches, unless my pre-determined master theme allows for mis-matched pieces. I'm not terribly concerned that it isn't the best sword in the game, so long as my sword skill is maxed and it looks bad ass with the rest of my equipment.
5. Ammunition is life, so I must reload. This goes as far as to emptying a seven shot clip in an M1 Garand because "screw you, that one bullet could save my life!" Even before I put a weapon away it has to be a full clip or I get the niggling feeling in the back of my brain that something isn't right.
6. I have to conserve everything, because you never know. The best example would be Big Daddies. All that armour-piercing ammunition just kept flying past me because I always worried something bigger and meaner would exist that I would need those precious few rounds for. This made those fights so much harder and I face palmed so hard the first time I just splurged and used some unique ammo on a Big Daddy. This happens in RPGs too where I'll used every last MP I have before even considering using a health potion. I can always rest at an Inn, I can't just magically get potions!
7. My last is easily the strangest. Every single player RPG that doesn't have voice acting (which is becoming too few and far between now) I make up voices for the characters and I read their dialogue out-loud, even if nobody is around and enjoy it far more than having pre-set voice acting in a game. I don't know why I feel compelled to do it, but it's so much more satisfying to enunciate the emotion of the characters myself, like we have this connection now because of it. I even sometimes project personalities to characters that aren't even there just because the voice I impulsively pick gives them that impression (Asnabel from Ogre Battle 64 will forever be a disease riddled, dirty old hobo man because of this!). I even add sounds like Asnabel coughing wildly between dialogue or Cthulhu from Cthulhu Saves the World having slurping sounds from all those tentacles, but I may never, never! ad-lib dialogue for a character that has dialogue in a game. Sometimes I even just think about a clever line they might say or something different because of a trait I've projected upon them and I have to restart the entire conversation/section just because it doesn't feel genuine to me if I don't stay faithful to the text and mold the voice around it. The only exception to this, as was previously mentioned, would be characters with no dialogue at all, in which I will sometimes ad-lib clever responses to dialogue and end up having this really awkward conversation with myself. That last bit can extend to other games, too, but usually only beat um' ups and other very old-style type games. (Godzilla will always shout out "RAWRUAWRUAWRUAWRUAW!!!" when he breaths nuclear fusion over everyone)
...Damn I'm frigging weird. So many convoluted rules and habits! D=
EDIT
Just thought of another one. I'll always make a specialized unit in an RTS game some sort of quasi-hero. This seems to work best in the Command & Conquer series but can extend to having only one powerful hero in games like Heroes of Might & Magic. They're like my super commando and I send them in to destroy entire bases by themselves because they're awesome!