One thing that immediately struck me was that while the new skill system is...interesting (although it's actually even less intuitive than the last one) it has a new, weird limitation.
Skills that do not fall under a specific skill denomination *cannot* be bound to the mouse buttons 1 or 2 - even with the advanced 'elective' system enabled. This was especially noticeable on my Demon Hunter - I like Evasive Fire as my Hatred Generator and it works just fine. Under this new system though I can't use it as such unless I am willing to spam a number key.
It's not gamebreaking, but it's rather annoying.