Guovssohas on 30 March 2008
IMO only 3 things;
1. Less linear levels, more like in Mario64, where you could pick up any star in any order, which requires more exploration.
2. Bigger and more "important" hub world, filled with secrets, again more like Mario64.(yes my all-time fauvorite is Mario64)
3. More secrets, maybe something like the blue coins in SMS. And the more you find the more extras you could unlock. Or something like the 3 coins in each level in NSMB.
The game has some other small flaws but they are too small to mention here.
