Love the upgrade system (would have liked to have a bit of explanation in the beginning, but that's fine). Only problem I've seen is the glitch where I keep gettin' stuck in the islands above me.
I love the concept, I love the way it plays out. but the jumps that I keep thinking are impossible at the current stage are frustrating when I leave and then finally return 10 minutes later (to make 50 attempts before I get it)... those I hate.