If you have time, you should watch the whole Smash Bros Nintendo Direct from the other day.
https://www.youtube.com/watch?v=7xUWnQu2GrsIf you don't have time, here are the deets:
-The 3DS version will be released this summer, while the Wii U version will be released this winter
-The 3DS version will (mostly) run at 60 FPS, even while in 3D mode. Some Assist Trophies and Pokemon will animate at 30 FPS
-Both versions will have the same character roster, but different stages. Stages that appear on both systems may have different cosmetic designs
-The 3DS version will have 2 songs linked to each stage. The Wii U version has many more.
-Some stages have boss characters on them as stage elements. For example, the Mega Man stage will have the "Yellow Devil" which will attack players. The boss can be attacked and destroyed, and will become a large explosion attack for the player that delivers the finishing blow
-Both the Wii U version and 3DS version will have the same online modes. The Wii U wired LAN adapter is recommended
-When playing online with friends, matches can be customized however you wish
-When playing online with anyone, there are 2 modes: For Fun, and For Glory. In For Fun mode, stages are picked randomly from all available, except Final Destination. All items will appear, and only wins are recorded In For Glory mode, Final Destination is the only stage, no items will appear, and both wins and losses are recorded.
-Almost all stages also have a "Final Destination Mode," that reduces them to a flat Final Destination-like stage.
-Online matches are no longer anonymous; Nintendo Network IDs will appear
-A reporting system will be implemented with bans for system abusers, as well as bans for baseless reporting
-There will be no multiplayer ranking (though matchmaking will be skill based), however, there will be a ranking system for scores accumulated in Solo modes.
-New items include the POW Block, Beetle (LoZ:SS), Fire Bar (Mario games), Back Shield (Kid Icarus), Bombchus (LoZ), Fairy Bottles (LoZ), Ore Club (Kid Icarus), X Bomb (Kid Icarus...?), Hocotate Bomb (Pikmin), Rocket Belt (Pilotwings), and Steel Diver.
-Assist Trophies... not sure which ones are new. But there's a giant pong game that hits people.
-In addition to Poke Balls, Master Balls can now appear, and contain only rare/legendary Pokemon. Several recent generation Pokemon have been added.
-All characters that could change form mid fight can do so no longer. Therefore, Zero Suit Samus and Shiek have been confirmed as playable characters wholly separate from their counterparts. ZSS has been given some kickass rocket high heels (seriously, they're hilarious), and both Zelda and Shiek have been given new Down+B moves (Summoning a Phantom and an acrobat kick attack, respectively)
-Kirby has a new hammer attack and a new Final Smash where he attack the whole screen with a sword
-Dedede throws Gordos (the spike ball things) instead of Waddle Dees
-Lucario has been made stronger, and its aura attacks will get stronger as he receives more damage (which can improve to the point where you can accidentally fly yourself off the stage). It can also Mega Evolve, which powers his aura attacks up to maximum
-Olimar can only have 3 Pikmin following him at a time, and he will always pull them in a set order (Red - Yellow - Blue - White - Purple). His Up+B has been replaced with flying Pikmin, letting him fly across the stage, but he will fly slower if he has Pikmin with him
-Pit can no longer glide, but has been buffed elsewhere. His Final Smash has been changed to Three Sacred Treasures
-Yoshi is confirmed as playable (no really) and stands more upright
-Rosalina can send out and recall Luma with B. While Luma is at distance, Rosalina can command Luma to attack. Her Side+B is a projectile that can be used while they are separated, her Up+B is the Launch Star, and her Down+B pulls items toward her and dissipates projectiles. Her Final Smash is a shooting star attack that affects most of the stage
-Little Mac was designed to have extreme attacks on the ground, but very little power in the air, with both an attack power reduction while airborne and a weak Up+B recovery. He also has a power meter that when built up via hitting and being hit, will let him do a KO Uppercut, a guaranteed one hit kill. His B is a charge punch, Side+B is a jumping lunge punch, Up+B is a twisting uppercut, and Down+B is a counter. Final Smash is a transformation into Giga Mac from the Wii version of Punch Out. Wireframe Mac from the arcade game is an alternate costume
-Villager's B move is Pocket, which lets him catch and reflect projectiles, Side+B is a Lloid Rocket that brings him across the stage, Up+B turns him into Balloon Fighter, and Down+B grows a tree he can chop down. His Final Smash has Tom Nook build a house around people that explodes
-Mega Man's normal and special moves are based on previous Mega Man games. His Final Smash involves other incarnations of Mega Man
-The Wii Fit Trainers' Down+B is a charge move that powers up everything else, and B move is the Sun Salutation, a charge shot. Both the female and male Wii Fit Trainers are playable, and both are exactly the same, including height, power, and range.
-Special moves can be customized, but this can only be used in local multiplayer, or online multiplayer with friends
-The 3DS has an exclusive mode called Smash Run, similar to the City Trial mode from Kirby's Air Ride. All players go through a Subspace Emissary-like stage for 5 minutes and fight enemies and open treasure chests that contain power ups. When time is up, all of the powered up players are taken to a regular stage to fight each other.
-Finally, both Charizard and Greninja are confirmed as playable characters