Do you own it? How far into it are you?
For me what makes it go from good to great is the sheer variety of gameplay mechanisms and environments. The ideas they came up with for levels based on manipulating gravity and other fun things brought about by the galaxy theme are really great and well done.
What all this boils down to is constant fun, the further you go into the game the more new things you see. Skating on ice with mario and doing jumps and stuff is brilliantly fun. A fireflower in a 3d mario game? Again tons of fun. I don't wanna ruin anything else, but its constantly giving you a new experience so the game doesn't get boring.
The graphics are great, the music is epic and nostalgic all at once. It's pretty damn high quality mate.