Everything regarding drawing characters. I still have a lot to learn and all, but I am a lot better at proportions, anatomy, perspective, drawing poses, expressions, hands and specially SHOES, than I was some years ago.
I also kind of struggle with legs and some kinds of clothing, but I know I can pull all of that out if I spend enough time (and energy) on it. And I'm still pretty bad at actually painting, as in... not just coloring, but painting painting.
This is one of my favourite things about drawing characters along with facial expressions. I usually find this to be even more important than the design itself. Doing it right can turn a fairly crappy and boring character into a very charismatic and lovable one. As long as the design is not, you know, impossibly hard to look at.