I haven't played New Super Mario Bros. 2, so I can't speak to that.
To some extent, all developers are guilty of rehashing ideas and mechanics that worked well the first time around. There are some exceptions -- Jak II comes to mind -- but the majority of franchise games don't break with tradition.
I just can't see people blaming Nintendo for rehashing its ideas and producing lackluster games when many of the most recent first-party games are among the best-rated Nintendo games.
Donkey Kong Country Returns (2010) -- 87/100
Super Mario 3D Land (2011) -- 90/100
The Legend of Zelda: Skyward Sword (2011) -- 93/100
And that's 25-30 years after the first installment. That's pretty damn incredible.










