Doesn't really matter what they are... Just things that don't have much bearing on the overall game. For example, here are a few of mine:
The Legend of Zelda: A Link to the Past (SNES):
-Link's sprite has pink hair... I'm sure I'm not the only one who noticed this. Surely this wasn't a palette restriction?
Resident Evil (Playstation):
-In the original game, I noticed that the characters (Chris and Jill) shift their feet slightly while aiming and turning in place. It's a pretty pathetic animation, but in RE2 and 3, there is absolutely no animation while doing this. It's as if it was too small of a detail to even be concerned with in the following titles, or that they just didn't care to make a better animation.
-If Olimar is standing on an incline, nothing happens, but if you press d-pad down and lie on the ground, he will begin to slide.
Tony Hawk's Underground (Gamecube):
-Of all the games in the series, I feel Tony Hawk looks the least like his real-life counterpart in this particular title. Also, the character models seem weaker overall than the other games in the series (relative to their consoles, of course).