Something about that sonic render on the back looks very odd to me. The background looks completely 2d, in an angle that parallels the box itself, yet Sonic is standing as if there is a floor under him. It looks strange, especially with the shadow casted behind him.
That may just be me though. Please keep going with this. I'm liking where it's headed.
It's looking really nice, but the Sonic render still seems odd to me. I do agree that one probably should go there though. I suggest you swap it with a render of Sonic (or some other character) jumping or flying. Right now it looks like he's standing on an invisible floor.
That render looks much, much better. I really like where this is headed, and I want to help make it better, so please don't hate me too much if I'm always suggesting things . Right now I think the tagline is a weak part of the back. The red banner with the basic font works well as a subtitle, but as a tagline it seems too boring. I would suggest you do something more similar to the main logo. Tagline's aren't exactly something I'm good at, so I can't give you a very detailed idea of what should be there, but I think it needs to be a little flashier.