Meta Knight's ↓+B move, if you hold down B, is an attack. When you don't hold down B, it's just to get you farther to places.
Zero Suit Samus's ↓+B allows her to get an extra jump. If you press B in the air again, she'll kick.
Diddy Kong's Side+B can be used in another way. In air, if you press B, Diddy will kick.