New Characters:
Baby Mario and Baby Luigi
Wario
Waluigi
Toad
Bowser Jr./Shadow Mario
Diddy Kong
Raichu
Wigglytuff
Igglybuff
Individual character entries. (the absence of those entrances was one of the only things that disappointed me on SSBM)
More items:
Green Mushroom (1up in player's life stock)
More Stages:
Delfino Plaza
Noki Bay
Roof of Luigi's Mansion
(All past stages)
Ganon's Tower (LoZ: OoT)
Mt. Battle (GCN Pokemon RPGs)
More Trophies
Wart
NES superstars (8-bit sprites of Mario, Link, Samus, etc.)
NES baddies (8-bit sprites of King Koopa, Ganon, Mother Brain, etc)
new cheers
And as said before, a more apropriate voice for Luigi. Almost everyone who has played Mario Kart or Luigi's Mansion knows that Luigi's voice is lower pitched than his brother's.