NSMB U has the 2nd best level design in the entire 2D Mario series, with only Luigi U surpassing it. Plus what's this about not getting good until too long? The challenge picks up sooner in NSMB U then any 2D Mario since Bros 3 with only Luigi U surpassing it again.
I've talked about it before in my Run the Series threads, specifically the first one on 2D Marios. Looking back at that thread, I see you had NSMBU and Luigi U up there in the 2 and 3 spots of your personal rankings. For me, NSMBU was 6 while Luigi U was 3. I gave my reasons for NSMBU there but I'll reiterate them here:
#6. New Super Mario Bros. U - I realized one day that I had 3 Mario games I hadn't even played in my collection which would have seemed unfathomable to my young self who would have wanted to play any new Mario game he could. The games were Galaxy 2, NSMBU and Mario 3D World. I went with Galaxy 2 expecting to dislike it the most since I've long been on record of not liking Galaxy 1. Turns out I was wrong about that and Galaxy 2 might be the best Mario game I've played since Sunshine. Anyways, I followed that up with NSMBU so maybe that has a factor of why I'm not as positive as other users on this game.
When I first opened and started playing this game, my cousin was visiting with me so we played through the first world and some other levels together. As I was playing it, I was locating star coins or hidden blocks like those beside walls at the door to a castle boss. My cousin couldn't figure out how I knew where to look and find these things when I was playing this for the first time. Thinking about, I just knew what to do from having played all the NSMB games and other Mario games up to this point. You've just long been trained on this stuff as a player. Yet, the more I played the game after he left, the more it stuck with me because I just wasn't finding the game super fun and was wondering if it was because of all the other Mario games I'd played up to this point. Was the series just too repetitive now? Was Nintendo too stuck in sticking to fans expectations? Could they change things up or would that be rejected? That was my headspace through the first 4 worlds.
Finally, in World 5, when I encountered a sort of repeat to the Forest of Illusion from SMW, I discovered the secret level through the Ghost House to the Parabeetle level and the game suddenly started opening up again in fun and challenge. There was the strange Van Gogh art level right after and I began to enjoy the game a lot. It definitely had a lot of hard secret exits and I ended up having to use a guide to find them which I hadn't done for a Mario game in a long time. Not sure if that's a good thing but it just seems to symbolize the unevenness this game had for me of being half blah and half really fun. It's just a shame it took so long to reach the fun point. I think SMW still has a better cohesive and put together overworld but it was nice to see that return ever so briefly in this game. But after this game, I also share the opinion that it is time to put the "NSMB" series to rest especially when the DLC expansion pack manages to outrank the main game.