It certainly has not failed, and that is not the point of this thread. Not a total doom and gloom thread, but you are almost right. There is really nothing else to want if you have most of the third party ports already, and it is just another good Mario game. Did it get rave reviews like Galaxy 1 and 2 where Nintendo actually put in some effort? No. Is it a good game that would be a nice purchase with your wii U at some point? Yes.
I can't speak for what reviewers saw in the game, personally I think the game is excellent. In my opinion, it is better than New Super Mario Bros. Wii. The power-ups (i.e. Acorn Suit) and Yoshi's most remind me of the gameplay from Super Mario Bros. 3 and Super Mario World. The World map looks like a fully fleshed out World for the first time since Super Mario World, rather than the seemingly randomly boardgame style connected dots of NSMB Wii and NSMB 2 3DS. Having played NSMB2 on 3DS, I have to say that the two games are completely different. I liked NSMB2 just fine, but I am not completely hooked by it the way I was on NSMBU from the very first stage. So, I completely disagree with the notion that NSMBU is a tired rehash of overused 2D platforming gameplay. Besides the fact, as far as consoles go, New Super Mario Bros. released on Wii in 2009. Super Mario Galaxy 2 released on Wii in 2010. So, I don't think it was overabuse to release NSMBU on Wii U before another Galaxy title comes out anyway.