I actually like what people call the NSMB formula--great graphics, great level design, great mechanics, great gameplay, great presentation. As such, I don't think the conservatism of NSMB2 was bad by itself, but how and where they chose to emphasize it. I'm especially talking about the boss fights that aren't Ludwig--they're either too easy, too boring, or both. Iggy's boss fight is literally a rehash of his fight in Wii, and it's not even made tougher or more interesting. In Wii, it had the element of taking place in a tight space with a huge Chain Chomp that moved around unpredictably and got bigger after the second hit, so you had to be careful with where you went, lest you get hit. In 2, those elements are removed, making Iggy much easier to hit. And it's not just the boss fights--the levels are often too easy and combined with the insane amount of coins in the game, you'll virtually never risk getting a game over.
The sad part is that the game actually does have moments of genuine creativity and innovation--just look at the levels in Worlds Mushroom, Flower, and Star. World Flower-1 is set against a sunset backdrop on a mountain and features snake blocks interacting with pipes. Flower-Ghost House features an underwater section where you're being chased by a gigantic ghost. World Star-1 is set at night and has switches that move block formations. Star-6 is an underwater level that takes place in a castle in a haunted forest. There's so much here that's creative and works well. Heck, the very idea of collecting huge amounts of coins is actually genius for the game to center around.
But the game doesn't build upon these elements and instead opts for recycling old content from Wii and DS most of the time, even downgrading it. Luigi has to be unlocked by beating the game. The Spin Jump is gone. The multiplayer requires both players to have the game and makes the camera centered on one player at a time. Coin Rush needs to be bought when it could have been unlocked by the in-game coin counter. But by far the biggest flaw is the complete lack of Wario anywhere. For a game that emphasizes coin-collecting and greed, Wario would have been a perfect fit as a playable character, complete with additional moves that complement his style, and unlocked after certain conditions were met. The game could have added him in, brought back the Spin Jump, fixed Coin Rush and multiplayer, and allowed Luigi to be playable from the start, all without compromising its identity. Just look at Super Mario 64 DS, which ironically also has Wario as a playable character. That game honors and preserves the core elements of the original Super Mario 64, yet manages to establish its own identity. Adding Yoshi, Luigi, and Wario and giving Yoshi the spotlight in the beginning of the game, superior graphics, new stars, new requirements for existing stars, new ways to play existing stars, new levels, even minigames, the game revolutionized Mario for the DS while maintaining its ties to the original game. New Super Mario Bros 2 should have focused on building on the DS and Wii games instead of copying them, and it should have expanded on its positive elements instead of downplaying them. Heck, adding Wario alone would have given the game a better reputation.
Now, what does this have to do with U? Well, U seems to try to build on what Wii established, adding Baby Yoshis on top of the regular Yoshis, having all four characters return, and the inclusion of Nabbit. But it doesn't do anything truly groundbreaking enough to justify being released just months after 2. It doesn't add any new moves, doesn't build on the traditional level themes, and its story, while an interesting take on Mario, feels minimal. U feels like it's trying to make up for the lack of ground that 2 gained, and as a result, even though it's a genuinely good game, it feels unable to truly rise to the level of legendary status it should have. Had 2 done all those things I said, it would have gone from a good, but flat, game to truly great, thus continuing the series' iconic track run that started with DS and Wii, thus allowing Mario Bros U and Luigi U to get even better and become truly great in their own right, and thus ensuring the entire series would be renowned as legendarily as the original 2D Mario series from Super Mario Bros to Super Mario World. Heck, we might have gotten even more New Super Mario Bros games later in the decade, and even unto today.