About what you is referring specifically? From a gameplay side? Until now the only bad thing for me is that the function to upload video on facebook doesn't work.. 
Free Step Dodge
Kronin
Zeo
As far as online goes, the Lobby system is busted now with the option to have 5 bar lobbies only considering 95% of the game's population is typically 2-4 bars, nevermind people's connections sometimes easily dropping from 5 and getting autobooted from the room.
Zeo
Then there's the absurd ranking system that displays all your information so anyone that want's to avoid you can. On certain days those 2 things can effectively prevent you from being able to play anyone. Then there's the replay upload functionality on 360 on top of the old replay glitch still being there...
Zeo
As far as gameplay goes, bringing up the flaws would be beating a dead horse, so I'm not going to touch on that now. Oh yeah, and no music plays during the Zack Island stage, awesome.