I've seen the demo in action (on IGN Wii) It's got 14 characters and 11 stages.
Mario
Donkey Kong
Link
Samus
Fox
Pikachu
Diddy Kong
Pit
Meta Knight
Ike
Peach
Yoshi
Sonic
Bowser
As for the Stages:
Battlefield (a revamped version of Battlefield from Melee)
Delfino Plaza (from Super Mario Sunshine)
Bridge of Eldin (from Twilight Princess)
Norfair (from the Metroid series)
Yoshi's Island (from, well, the Yoshi's Island series)
Halberd (from the Kirby series)
Lylat Cruise (from the StarFox series)
Pokemon Stadium 2 (from the Pokemon series)
Castle Siege (from the Fire Emblem series)
Smashville (from the Animal Crossing series)
Skyworld (from the Kid Icarus series)
Now, all they gotta do is mail me a copy and we'll be all set.
Cordac's teardrops plunge into the abyss with nary a splash or spray
And strike the lake miles below, exploding outwards into the fray.
March 9, 2008?!?! Okay, now you're getting Chaotic (in the bad kind of way).
"I believe in you Worlder! Don't fall to their side..." -- Pocketkoi