I'm weak in the hands, thanks to carpal-tunnel syndrome.
Drum sounds I'm definitely weak on, especially kick.
Also, getting a mix to translate. I do many many mixes and check on many different places before I give it up (I don't think I've ever been 'finished' with a mix, I just reach a point of diminishing returns.) Unfortunately, there's not a whole lot I can do at the moment to better myself with this, since I'm sure part of it is the room I mix in, and the monitors I mix on. But practice, that's free!
And mastering. I attempt a little mastering for what I post on the internet, but for albums I'm more than happy to pay someone to do it right. And I don't really mind too much that I'm not good at mastering, because I'm not really interested in it.