Uh... costumes are better overall. Dark Link, Corrupted Samus, GOLDEN BOWSER, etc.
The game is like a mix of Melee and the original. Now for a paraphrase of someone elses info.
-Stages are smaller like in SSB
-Items and Assist Trophies make the game insane
-Mario is better than Sonic overall (who is now the fastest running character, with average attacking speed)
-Ike is uber-slow, but really strong
-Donkey Kong's about the same, and Diddy is good in general
-Fox's Final Smash (Land Tank) can DO BARREL ROLLS
-You can USE BOMBS WISELY now (the StarFox Smart Bomb is in and huge)
-Mario is also a combination of his Melee self and the Doc, with some new moves
-Each character has three taunts, one for each D-pad direction
-Pikachu's Thunder (down-B) is a kill-move now (and one of the strongest)
-Latios/Latias is in as a Pokeball
-Peach's Final Smash puts everyone to sleep for about the same time as Jigglypuff sleeps after using Rest
-You can't airdodge in a direction, so wavedashing is out
-C-stick automatically charges smashes now and does tilts if you push the direction on the both sticks
-Sonic's dash dance is even better than Capt. Falcon's
-Fox doesn't fast fall so bad anymore (no more insta-kills for Yoshis that egg you off the edge)
-Catching items out of the air (with Z) is a lot easier now
-Wall jumping still in (even off ceilings)
-Metaknight's teleport is really fast; he has a fast roll as well
-Shines are much faster in the air, and I guess it's still possible to shinespike, just harder to kill them with it
-Footstool jumping works beautifully (jumping on someone's head to spike them)
-Crawling's there
-Personal character intros in
-Bowser walks out of fire
-Mario uses pipe
-Arwing drops Fox off
-Bombjumping (Samus) is harder
-Mario's FLUDD doesn't do damage and stays on his back until you put it away
-Fox's laser doesn't go very far
-You can stand on boxes and hit them in the air (a la up-smash) without breaking them
-After Link's Final Smash, the Triforce breaks up into golden pieces that hit and knock away anyone in front of him
-His bombs kill in Super Sudden Death
-Fox's Illusion (over-b) can still be shortened
-Walking and shooting with the ray gun (or any gun) is awesome
-Moonwalking is in
-NEW TECHNIQUES
-Hugging: you can walk, dash, roll, and slide off the stage and directly grab the ledge (go edgehogging!)
-Ink Drop: Do a failed dash dance (dash too far and then turn around) and you will just fall down; you can cancel falling with throws and attacks, though, so it's possible to dash right past someone and then instantaneously grab them)
-Peach's whornado (down-smash) is less abusable now
-Bowser is actually good now
-Mario
-Feels like 64 version
-Great combos
-DK
-Basically the same
-Samus
-Missles are weaker
-Link
-Arrows faster
-Fox
-Nerfed
-Peach
-See above
-Yoshi
-Egg-tossing gives *small* vertical distance, but can be done over and over
-Bowser
-Better; hard to kill
-Pit
-Average, but great recovery
-Metaknight
-ABUSE: Quick and powerful, with nice combos and great recovery
-Diddy Kong
-Combo-happy too
-Ike
-Way strong and slow; doesn't flinch easy
-Up-B is straight vertical, like Luigi's
-Sonic
-fastest character in movement
-awkward in attacking (at least with the Classic controller and Wiimote)
All credit to DarkGea (GAMEFAQS) and Gimpyfish (SmashBoards).
http://smashboards.com/showthread.php?t=121329&page=1