That's not really fair.
The Mario series has a history of making radical changes to itself with each sequel.
Up until the Wii, only one sequel had been made in the same mold as a prequel. SMB2 for the Famicom was a direct sequel to SMB, but in the US, this was not the case originally (SMB2 in Japan was released as The Lost Levels).
SMB1 was completely different from SMB2 (US) which was different from SMB3. Then you had Super Mario World and Super Mario World 2 which were completely unique games that had little to do with each other or previous Mario games. Same can be said of the Gameboy titles which were all different to the point that they could have been from different series.
Once N64 hit we had Mario 64, which was obviously new and different, and the same occurred once again with Mario Sunshine.
All of those mainline Mario games were wholly unique entries in the series.
Now, with the Wii, we had both New Super Mario Bros and Mario Galaxy 2 which look and feel very familiar. NSMB was created in the mold of the DS game while Mario Galaxy 2 is a direct sequel to Mario Galaxy.
I don't believe this is a bad thing at all, honestly, as those were great games...but it definitely isn't the norm for this series. It's unprecedented!
While I understand what you mean, I actually feel this is very impressive. To execute so many ideas so perfectly over the course of a game is not an easy task. Most developers don't even TRY to implement so many unique levels into a game. I would certainly have loved to see them take some of their ideas further, but at the same time, I also appreciate the fact that there ARE so many different great ideas in there.
Job well done by Nintendo.