I got NSMBU with my mariokart 8 bundle and completed it last night. It very much reminds me of Super mario world in terms of level design and, if you ignore the comparative freshness and very strong nostalgia of super mario world, NSMBU, to my mind, is the better game and a huge improvement over the previous entries in the NSMB series.
There were some minor things I disliked. The graphics could have been fancier. They did the job and had nice backgrounds but overall it looked unremarkable. Mario was too shiny. Despite getting so many lives that they're never an issue it still forced an annoying save system on you which requires you to remember to save before you quit or you lose progress. an auto save like in SM3DW would have been far better. I'm also irritated by the constant prompts to post to mii verse.
Like Donkey kong tropical freeze, it's a short game. Yet the ease of collecting all the big coins to unlock levels makes it a far more comfortable game to attempt to 100%. I found the idea of 100% DK:TF a little daunting, bearing in mind I have many other games to play right now, and I have barely picked it up post victory. I'm far more likely to keep coming back to NSMBU. The speed run challenges are lots of fun too.