I'm confused by the disappointment... wasn't it already announced that the Wii U Mario would have a style similar to 3D Land? Heck, I've even referred to it informally as 'Super Mario 3D World' because of this. I'm only a little surprised that it's turned out to be the actual title and certainly am neither shocked nor disappointed by the way it looks.
3D Land is probably the most underrated Mario game ever. Which is Nintendo's own fault, since it only really gets going in the 'extra' worlds and by that time a lot of players might have given up. But go it does, from that point onward.
If the leap from 3D Land to 3D World is as large as the leap from NSMB to NSMBWii was then this is a classic in the making. And it might be: four player support is a fantastic indication of that.