Every stage introduces something new. Whether it's a level obstacle, a move, an enemy or an item, there's always something else being grafted into the mix to keep you on your toes. My first playthrough had me skipping a couple of worlds with a shortcut, which I loved.
Tons of content to play through; Clever level design; All the characters feel distinct
Music never hooked me; Graphics are a mixed bag; Little incentive for double dipping