Rolling jumps! (crouch, then press B+A simultaneously)

Too bad levels somewhat obstructs using this ability to breeze through them as fast as you can, like in 3D Land.
Cat running animation in level select!

Cat Mario runs completely different in level select from regular gameplay. He's walking like a very proud and smug cat with tail up vertically in the air. It's so funny and becomes funnier when you run because animation just speeds up to 2x.
Music remixes:

Sound effects and sound design:

So many little things: birds chirping and flutter of wings when they fly away, cat goombas meowing, muted music when you sink into water.
Fire Flower Peach:

Got damn.
What are yours?