I don't own a WII U yet, but I'm saving up. All of the Mario games are an incentive but there are just so many awesome games on this system. Plus, the WII Network is a lot better than the network on the WII.
Pretty much all 1st party games are fantastic. Always have been. Like most other Nintendo consoles, it is all about the 1st party games as 3rd party support continuously struggles.
Pretty much exclusively for the new Smash Bros game. I just couldn't miss it, even if it costed me ~£200. Also the prospect of a new open-world Legend of Zelda game is mouth-watering. Can't wait!
You know what? I never bought the Wii U. The only reason this is in any way significant (because given the sales numbers MANY people didn't buy the Wii U) is because I own EVERY (I mean every) other Nintendo console/handheld gaming device. It just didn't interest me enough.