I am going to have to go with Pap on this one, while NSMB was a very good game, it dissapointed me as well. Personally I don't care much about the challenge of games, so that is not really my complaint. My complaint towards NSMB is that it felt like a step back, even from SMB3 in alot of ways. It needed more item variety, better level designs with some neat little secrets placed here and there (a SMB3 whistle search would have been great). Boss battles were pretty uninspired as well, they didn't have much of anything going for them when it came to fighting. I hate to go back, but SMB3, even with it being the Koopa kids really made you feel like you accomplished something when you beat the bosses, this, you don't get that feeling. Even to this day my heart starts beating when I am facing a Koopa Kid, especially the later ones, but NSMB, I'm not getting that.
Level design feels quite basic as well, and I wish they would have put more effort in character designs. I love Nintendo's nostalgia but not to the point of reusing so many character models from previous games (Mainly SM64DS), I want to see some new designs for classic enemies. Finally I found Koopa Kid to be a bore, and nothing to get excited about. What was worse was the final boss fight, which IMO, pales to most of the final battles with bowser in both excitement and creativity.