The only things that made me not adore NSMB2 were the recycled environments and music.
Mario is as much about "wow" inducing and magical level design as it is about straight up smooth platforming. The NSMB titles have this stale sigma, and somewhat rightly so, because they haven't really strayed too much from the first title in terms of environments and variety of level design. NSMB2 is most guilty of this.
But honestly, though, NSMB was a great revival, and NSMBWii was a fun sequel that pretty much brought about the modern use of multiplayer in platformers.
I think NSMB2 had to bring something new gameplay wise, or just have really inspired level design (ideally, both). The gold affixation wasn't really enough for most, and rightly so. The game is still a blast, but it suffered from bad timing, coming out so quickly and so closely to NSMBU.
It probably would've been best to delay NSMB2 for awhile and really put everything they had into it. Release it in late 2013, maybe.
It would've put some distance between it and NSMBU. The two launching so close just makes the "rehash" argument easier.
Ultimately, though, NSMBU looks like it's really a breath of fresh air, even though I think the series has only had one really uninspired entry (NSMB2). So maybe breath of fresh air isn't the right term. I think it's looking really great and original, and I think it'll hush the naysayers of the NSM series.
I think it'll probably be either NSMBU or Rayman Legends for me at launch. I've got NintendoLand with the Deluxe SDK, so that's a pretty awesome launch to me.