I'm noticing a pattern here with Nintendo games now. Of the last three Nintendo games that I've bought (Link Between Worlds, Pokemon X and Super Mario 3D Land) all of them have been extremely fun and fantastically designed but have one fatal flaw in that they are too easy. Each world in this game would take about 45 minutes to an hour for me to beat and when I beat the game, I was left with 131 extra lives. I realize that Nintendo may be trying to reach a younger crowd with this game, but they shouldn't forget about the older Mario lovers who would like a challenge.
The only other downside in the game was the repetitive nature of each level and the bosses. The individual design of each level was brilliant, but as you went on in the game, you started to get the feeling that you've been through this before. That feeling was at its strongest when fighting the bosses. There are three different bosses in this game: two nameless Bowser-lings and the King Koopa himself. You fight each one three times and each time you fight them, you use the same strategy of jumping on their heads and dodging them when they retreat into their shells and try to hit you. Bowser is even less of a challenge since you never directly fight him; all you do is dodge him on a bridge, reach the end of the bridge and jump on a switch that plunges him into lava. Rinse and repeat each one three times and you have this game.
Now I don't mean to dissuade anybody from buying this game. When I was playing it, the game was fantastically fun and after you beat it there are some challenge courses that amp up the difficulty. The levels are engaging with another great soundtrack and the return of the tanooki suit brings joy to my heart. The 3D nature of it also brings new life to the game so it still maintains the side-scrolling nature of its roots but it isn't chained down by those as you can explore up, down, left and right.
All in all, if you are looking for a challenging game, you've come to the wrong place, but if you ever have an hour to kill and you just want to spend it relaxing with a good game that doesn't get you throwing things everywhere, this is a great game.