For starters, it adds good ol' Yoshi to the mix. Oh, and the Cloud Suit--which is, in my opinion, the series' slickest power-up since Kuribo's Shoe.
I could spend hours on this stage...
Also, it eschews the last game's confounding "Comet Observatory" hub in favor of a map system that's similar to the series' more traditional, two-dimensional efforts (Super Mario Bros. 3, especially).
Finally, part two is, thus far, decidedly more difficult than its pushover of a predecessor.
That said, I'm only a few galaxies in at this point, so it's possible my opinion of the game will change as I delve deeper into it.
Are any of you also enjoying Miyamoto's latest masterpiece?
See also: 'I ♥ Mahito Yokota (and so should you)'