While the main story line of Super Mario 3D Land is admittedly rather easy, the Special World stages really pick up the intensity, in my opinion. I'm pretty far into the special stages (and would be further if tests didn't exist), and it's becoming apparent that this isn't a simple cakewalk. Again, it's not as difficult as some of the other Mario games, but it does a fantastic job of being challenging, yet not overly frustrating. This game is a must-have for 3DS owners, in my humble opinion.
Also, Boomerang Suit needs more hype. It's a lot of fun to use and can lead to some pretty silly situations.