The one that annoys me is with Igni flamethrower. Not sure if bug or WAI, but enemies don't seem to notice it until they've been in the flames for about a second. Can't tell you how many times I've started to flambe a guy only to have him run over and smack me, even though the graphics show him being on fire.
Also, does anyone else think that most enemies are too "good"? I'm not talking about combat difficultly in itself, but the way that any random wolf, drowner, nekker, etc can dance and dodge around almost as well as the witcher they're fighting. Not to mention how good every human enemy is at parrying blows. It's a small annoyance but I wish they had done more to balance combat while still making Geralt feel like the mutant, superhuman, master swordsman freak that he is.