I felt the same way for the first 4/5 worlds as you did. And looking at the map, it appeared as though the game would be far too short.
However, when you get to World 5/6, the difficulty picks up and the game improves exponentially.
I don't think it was GOTY (for me that honour goes to W101), but it does become a brilliant game. It's just unfortunate that the learning curve is so shallow. But then, this game was designed to be accessible, Christmas Day I was playing the multiplayer with my girlfriend's dad and sister who'd never touched a 3D Mario before.
Stick through the early worlds, relish the ease of it, because when you get to the endgame it's going to kick your ass!