I can't play ranged DPS to save my life.
My hunter is terrible. It's not his fault - he's well geared and he has an awesome pet worg - but the only reason his DPS is even respectable is that it's hard to do poorly at level 61. Frankly, I'm just the most atrocious hunter you've ever seen. Even in dungeons I'm completely familiar with, I make rookie mistakes on him constantly. I forget to turn Pack off. I leave Growl on. I don't reapply Serpent Sting. So far I've managed to not tab target over and pull other adds into a group pull, but that's mostly because I don't like to tab target.
Seriously, it's kind of nuts how terrible I am at this hunter. I picked up shaman, paladin, and DK with a lot less effort than this guy. I'm objectively trying my hardest to get good at this, and I'm not pulling it off. And it's very frustrating.
So let's turn that around - any classes or play styles you just can't master? Are you a clutch healer and an awesome DPS, but tanking eludes you? An awesome tank and raid leader who can't heal for beans? Are you a druid who can do anything... except figure out that kitty spec you keep wanting to play?