There has always been something about the NSMB series that just doesn't do it for me.
Control wise I've always felt that it lacked the precision of SMB3, Mario World, and even Yoshi's Island. It's much more in line with the original Super Mario Bros, which honestly doesn't control that well in comparison, but with some of the functionality of the 3D Mario games in place. While certainly much more responsive than Little Big Planet, it has always given me that same feeling. Simply controlling Mario isn't as much of a joy as it should be. Also, the addition of waggle in the Wii version actually dates the game a bit and detracts from the core Mario gameplay. I just don't much care for the power-ups in that game.
Visually the series isn't ugly, per se, but it has a generic "CG Mario" look to it. The backgrounds lack both the charm of pixel art graphics and the more artistic 2D art we've seen in a number of other 2D platformers. It just feels sterile and safe. It's ugly in the same way that Sonic Rush was ugly. They simply can't match the style of their 16-bit counterparts nor do they push boundaries. I hate to call Mario "generic" but it DOES feel that way to me. That said, I do think the WiiU game looks quite a bit nicer in this regard.
Audio design is one of the areas that has annoyed me the most. Who remembers the music for Mario World or SMB3? Everyone knows SMB1. SMW in particular used just one primary theme yet managed to deliver a very interesting soundtrack that set the proper tone for each type of level. Nintendo's ability to produce music has never been quite the same since the N64 era and only in a few recent examples do I feel that they made strides. NSMB, however, has been one of the worst offenders. I simply cannot enjoy any of the music AND the samples used are atrocious. The fake "voice" samples used so often are beyond grating at this point. I don't feel as if they've taken any steps to evolve this as each NSMB game basically sounds like the previous one from what I've heard. It's one of those games that I will play with an alternate soundtrack playing instead.
I feel as if Nintendo has begun to iterate just a bit too heavily on new Mario games rather than attempting to deliver something new. All of the NSMB games feel very similar to one another (mechanically, visually, and audibly). Even Mario Galaxy 2, which is incredible, feels a whole lot like more Mario Galaxy 1. Nintendo DID follow this strategy back with the Japanese release of SMB2 for the Famicom, but since that point they've constantly pushed boundaries with every new Mario game.
The 2D New Super Mario games look sterile. When I played the first level of Sonic Generations, I was blown away by the graphic design (using OG Sonic).
NSMB looks like a turd that Sonic pooped out. Not that it affected sales in the least.