Finally (I mean, FINALLY) putting Wario and Pit in the roster.
Not only giving the Ice Climbers a non-fail stage, but giving them one of the best stages in the game.
Co-op everything, including SSE. It was the only thing I liked about the adventure mode.
The music. The graphics. Best on the Wii by far.
The dev team realizing that if Snake makes it into the roster, then Sonic, of all 3rd party characters, absolutely needed to be in there. Sonic was the one 3rd party character that deserved to be in the game (should 3rd party characters make it in, I should say) if only for nostalgia's sake. Mega Man and Belmont are a close 2nd and third respectively.