Well a major problem right out of the gate is the lack of the gamecube controller which leaves us with : Wii remotes, classic controllers, and wii U controller.
I guess I will have to find a USB Gamecube controller or something because I and most people I know are garbage playing with wii remote (is uncomfortable and lacking in buttons), and the classic controller is just kind of awkward ... not bad but just not a solid controller. The Wii U pad (from what I can tell from just holding the thing at E3 and not using it with a game) is heavy and even more of a pain to use than Wii remote.
I want to believe that Nintendo will fix online with this one, but there is no indication so far that Wii U will have anything approaching a decent online system, and Nintendo does not have a great track record in this area anyways.
Of course, I will still end up buying it, and I hope they bring in more characters while also keeping the old ones ... there is no reason to get rid of characters, and I don't care if they are clones of other chars.