The structure of the game was too old for me.
It's like the New Super Mario Bros, I have no interest in playing games that feels like the old mario games I already played 20 years ago.
Mario 3D world was a step backward, and also unnecessary. We had the Open 3D series (64, Sunshine, Galaxy 1&2), alongside the 2D side scrolling series (Mario Bros 1, 2,3 World, New super mario games).
And here comes 3D world which is basically a 3D version of the old concept.
You people just don't realise that when Mario 64 was in development, a game like 3D world would have been the safe choice, but instead they pushed way beyond that, and they made Super Mario 64.
So yeah, it's not a bad game, it's even better than the New super Mario series (which are for me the "worst" games in the series) but still, it's the "worst" 3D Mario made. Mario 3D games always pushed the genre, took it somewhere nobody thought it could go, they are truly masterpieces. Mario 3D world, is just another mario but in 3D.
BTW, when I say "worst", I mean "less good". To be clear they're all good games.