3DS coming this Summer, WIi U coming in Winter.
3DS and WIi U versions will connect somehow, but no details yet.
Online modes:
For Fun: All Items, random Stages, ony wins recorded
For Glory: No items, Final Destination only. wins and losses recorded. Final Destination has different "skins"
With Friends: customize everything

Global Smash Power ranking... a reverse leaderboard.
Custom Move Sets... you can tweak characteristics of moves.
Smash Run mode... 3DS ONLY multiplayer search a dungeon for 5 minutes, killing enemies for powerups to use in battle that follows.

No characters will change forms mid-match.
Zero-suit Samus has jet boots.
Zelda summons Phantoms.
Shiek has burst grenades, and new kick.
Kirby has a charged hammer attack.
Lucario "mega evolution"
Olimar only has 3 pikmin, order matters. Winged-pikmin recovery.
Many characters have "stronger" movesets.
Yoshi stands up straight.
Rosalina "gravitational pull" to pull in items. Luma will wonder around the stage on it's own if thrown.
Little Mac is horrible in the air. Power Meter builds up to give an insta-kill punch.
Wireframe Mac costume.
Charizard.
Greninja.