To start:
I'm spending a few hours rearranging my displayed collection.
I find that I really hate Ball Joints.
It's simple and all, although I haven't brought myself to try the clear nail polish "fix" yet.
Notably, the ball joints on the exo-suit which came with MP Bumblebee. I spent 10 minutes trying to pose the guy -- Spike or Daniel, or whomever it's supposed to be -- because the arms kept popping off. Annoyed, I am.
It also happens frequently on the CW and PotP bots... It's not as bad, 'cos they're bigger, but still annoying.
I suppose it's worse for me as my toddler thinks it's fun to pop the legs off...