I think it's the lack of any real jab. Sure, the buster is what's expected and makes perfect sense, and on top of that it's an entirely unique character design. And, admittedly, I don't really know how the buster attack works, but a big part of characters I like are having quick, close range jabs, usually that provide some good hitstun spacing. Think of it like... a panic button.
On top of that, he looks... sort of clunky? I think that's the best way of putting it. He looks stiff, similar to how WTF moves, but in a less obviously comical way.
Don't get me wrong when I say I don't think I'll play him much-- I still think he's one of the most unique characters, he has an awesome design and it's executed really well. And I could be entirely wrong about him, too; it's entirely possible I'll enjoy him as much as or more than the characters I'm expecting to enjoy. But, as things stand, I don't see the appeal for my playstyle.