Personally the only Mario games I play for the story (or care about the story at all) are the M&L RPG games. That's because they are, well, RPGs, and thus the story is a big part of the game.
All other Mario games I've played I have played solely for the gameplay. I don't really play many Mario games too (haven't played Sunshine or either Galaxy yet) and not as often, so I don't really notice the repetition as much. The most recent platformer I played was NSMB, but since I hadn't played a Mario platformer since Super Mario Advance 2, it had a lot of new aspects (such as the Mega or Mini mushrooms) and was really fun for me.
In any case, I don't worry about story in platformer Mario games. The story is just there to justify Mario's presence and actions.
All other Mario games I've played I have played solely for the gameplay. I don't really play many Mario games too (haven't played Sunshine or either Galaxy yet) and not as often, so I don't really notice the repetition as much. The most recent platformer I played was NSMB, but since I hadn't played a Mario platformer since Super Mario Advance 2, it had a lot of new aspects (such as the Mega or Mini mushrooms) and was really fun for me.
In any case, I don't worry about story in platformer Mario games. The story is just there to justify Mario's presence and actions.