I haven't played as much as some others but I'll give my perspective.
Meteor Swarm on paper seems too strong - aoe and single target in the same skill.
The skills that have 3 'types' (eg the skill in the beserker tree, the skill in the shaman tree) take up too much space in the skill bar. Make each variant of the skill switch to the next when you activate it - eg fire resist switches to lightining which switches to cold.
The stalker polymorph in the ranger tree is incredibly boring with no skill use (other then barkskin/strength buffs when stalker is off cd). If you could switch out to use other skills it could be fun - I can imagine a stalker/traps build. If you do build a viable stalker it is going to be 40% (from memory) faster than all other characters....that's insane.
I have more but out of time right now.