I consider myself fairly high skilled with platformers, especially Mario ones. I find Lost Levels enjoyable and can clear almost any Mario level.
However, the skill challenges are downright brutal. Time trials, coin runs, 1up marathons... and I haven't even gotten to the Boost Rush ones, which basically force you to use this setup:

(That's a developer of the game, setting mandatory platforms down with the GamePad while he navigates the level with the Wiimote in order to beat it. This game needs the "For Super Players... ONLY" subtitle.)
Sixfortyfive on GAF is a much more skilled platform player than me and these challenges almost damn near broke him. I've barely scratched the surface and I'm already pulling my hair out. These are way harder than 95% of Lost Levels already.
Mario U is actually great so far. Same old game but the levels are quality and it can look nice at times. Has some of that Mario Galaxy sheen and isn't quite so flat and charisma vacuumious like the Wii game.
It's not terrible looking. The backgrounds are nice and it's finally, finally nice not to have my eyes cut open with jaggies.
Other than the music, all that's 'bad' about the game are the coin placements. I've gotten all in the first four worlds and most of them are behind a fake wall. You know, the completely innocuous looking walls that you can walk through. It means two things:
1) If you want the coins, sometimes you gotta just rub up against walls. The levels are well designed in general, but it doesn't have those suspicious looking areas where you feel the coins could be.
2) It's not skill based. I may be misremembering how it was in NSMB Wii, but between Galaxy 2 and 3D Land, some of the bonus coins were fun to get. They weren't just "hidden" to be found. Sometimes they were in plain sight and getting to them was the challenge. The coins in NSMBU are just pretty lazy so far.
The coins in NSMB U are a lot better than Wii. I may be misremembering myself but like 80% of those were "hidden wall tee hee!" ones. I was really annoyed with that aspect of the original.
Comparatively, U's coins have far less hidden walls coins, and those that do are usually somewhat clever at least.
And this isn't a positive or negative, but the 'seamless' world ain't like Super Mario World's. It's more like Mario 3 with the worlds connected.
The theme's are the same recycled junk from SMB3 but the structure is pretty much World to a tee, with a bigger emphasis on "1:1" secret paths and not secret paths that lead to otherwise completely-sealed off areas (see Butter Bridge, the lower right of Donut Plains, half of Forest of Illusion, etc.) In general I think this might be a little better gameplay-wise since you get more content without combing through secrets. Whimsy-wise World probably has the edge, although I will say I always did like World's multiple secret exits in some levels. Made you think you were completely done after finding the first one, but nope, more secrets to be found!
I also give an easy victory to the Ghost Houses of World. U's are better than most of the New series' but still not as clever as the originals.