No, I don't think multiplayer is the issue. Nintendo took this approach with 3DLand as well. It's really more of a focus of making a 3D Mario that is more "playable" and incorporates classic Mario elements. There's been many times when I was playing Mario Galaxy when I longed for the classic-style power ups (no life bar) and more traditionally styled worlds in a 3D Mario. 3DWorld gives this. There are also many people (especially women) that have a difficult time with 3D visualization when there is a roaming camera. A more locked camera perspective allows those people to play and enjoy a 3D game. I think the classic play and more accessible viewpoint are reasons why 3D Land has been a big success, and Nintendo wanted to continue this with 3Dworld while bringing in multiplayer, which is awesome.
I think the biggest problem with 3DWorld is in presentation. It's neither groundbreaking, nor does it have the "style" of Galaxy. The music is also nowhere close from what we have seen so far. Galaxy felt "fresh" while this game is more "classic". Contrary to what many say I think the visuals are better than Galaxy and the levels more detailed... they just don't have the cool space backgrounds and flipping camera angles.
I'm really looking forward to the game because it brings a lot of what I have wanted in a 3D Mario to life. I do wish the style or theme was a little more original though, and that the music and that orchestrated drama of Galaxy. The true game may have these things, but we haven't seen it yet.
