Me, I've never been able to play a Paladin to my satisfaction-- despite my obvious concerns (which have come up before and which I won't belabor here), the class just never vibed with my playstyle. Warlock is another one that, for whatever reason, I've just never been able to play past about level 10 or so. It's not that I think they're bad classes-- it's just that I personally just don't vibe with their playstyle.
What about you? Are there any classes that you just can't seem to play and get into? What's your anti-class?