#1 for me is track gremlins. You record the perfect track. It's immaculate, perfect, and completely awe inspiring. Then you come back to it the next day and the track gremlins have completely ruined your performance. Lots of flat and sharp notes, the dynamics are all wrong, etc. etc.
Seriously though: 1) Time, 2) Lack of practice, 3) Patience.
If I take the time to practice the stuff before I record the track, those pesky track gremlins aren't nearly so prevalent. Then I just have to have the patience to listen and mix, save and quit; repeat as necessary. My mixes always lack the sheen and shine that I'd like them to have when I'm done, but I just don't take the time. Maybe patience should be first on my list...