I disagree. I prefer substance over style. A game can look great but if the mechanics aren't there it's wasted potential. Look at LittleBigPlanet: Looks great, plays like ass. People complain about NSMBU, yet SMW continues to get praise despite being one of the easiest Mario games and being much blander visually in terms of level graphics it makes me think people would prefer Mario if it had no graphics at all.
Style isn't bad, though, and should totally be addressed. I personally think, at least with NSMBU, that they've hit a nice style for Mario. The clean look really fits Mario, and they did an especially great job on the level graphics with NSMBU. To me Lost World and 3D World are about equal in style. Sonic is going for a bit more cartoonish style and both are going for the clean look, with 3D World opting for more detailed textures.
And NSMBU does do new things, especially with star coins. One of my favorite moments in NSMBU was a star coin that required you to drop a koopa shell under water, stop it, wait for it to get back up, and then grab it under water to throw it at some urchins blocking access to a star coin. The Bowser Jr. battles are a treat as well. Then there's challenge mode, which is great.