From the looks of it, anyone who was Uber in Melee was toned down slightly, and anyone who sucked in Melee was boosted to Oblivion and back.
Here's all the information I've seen and collected over the days:
Mario is overall faster and more powerful
Fox has been nerfed. Now the lightest character in the game, and he's not as awesome as he was in Melee. His Up-Smash still hits hard though, but the reflector no longer has as much range as it did before, making it harder to "shine" someone.
Falco is Luigified. I haven't seen him entirely, save for his reflector, which is now thrown in front of him a short distance, instead of being used the same way as Fox.
Wolf playes like neither Fox nor Falco. His moves are entirely different. The only thing "cloned" was the Landmaster, for incredibly stupid reasons (Star Wolf never had a Landmaster, save for Assaults multi).
Toon Link seems to be in the same wind as Young Link was. Smaller, faster, etc. A few of his attacks are different, and somebody mentioned he had Bomb arrows, though I haven't seen those yet.
Link, if anything, took a slight nerf. His Up-B needs to be charged to do decent knockback. Haven't seen much else.
The great Captain Falcon took both nerfs and buffs. His knee is actually stronger and has more knockback now (someone reported 20% damage with it), and his Falcon Punch comes out faster (similar to the original SSB), and apparently his Falcon Kick literally takes him off the ground, letting him slide right over mines and what not. His others moves took a hit though, probably in the damage department.
Marth took a buff to movement speed. He's insanely fast. However, "tipping", or hitting with the ip of his sword, is much harder; the sweetspot is much, much smaller now. His Shield Breaker (Neutral B) now swings out horizontally with slightly more range, and someone reported it breaking a shield at roughly half charge. His Final Smash is an insane One-Hit KO. However, if he misses, he SD's instantly as he doesn't stop for anything except an opponent, and possibly a wall.
Bowser is much faster now, and apparently his standing Forward A attack has insane range and power. Overall, he's much faster and stronger then before.
Ike has been labled the "Newb Killer" Though incredibly slow, he's incredibly powerful as well, taking many opponents at fairly low percentages.
Pikachu was buffed to hell and back. His Downward Smash out-prioritizes almost everything. Standard B (Thundershock) moves much faster, and Down B (Thunderbolt) can be spammed somewhat. Basically, less lag and more speed for the electric pokemon mascot.
For Jiggilypuff, Rollout charges faster, Sing keeps opponents asleep longer, and Pound seems to have a little more range with a tad less power. Rest took a slight nerf, not doing as much damage, but it now also puts a flower on the opponents head.
Not much on Luigi. His Forward B (Green Missile?) has a larger arc apparantly. Alos, his Taunt was buffed from 1% to 2% damage. It also "pings" much like how Jiggilpuff's Rest did in Melee. A video was shown with him using it in training mode in the Luigi's Mansion stage, and the player that kept getting hit kept bounceing off the ceiling. In otherwords, it has real knockback now.
Peach took a nerf. Her Downward smash is much less damaging and has less knockback. She doesn't float as longer for her second jump, and her parasol doesn't go as high, but now floats longer with it out.
Pit has been labelled as nearly unkillable without literally KO'ing him into a wall; his recovery is that awesome.
Meta Knight has extremely good combos, but a lack of any good KO'ing moves. He has good recovery.
King DeDeDe has already been labelled as top tier. A heavyweight with 5 jumps and an incredibly 5 jumps makes him difficult to kill, and he hits hard.
Kirby got buffed massivly. Forward B (Hammer) now has much more range and knockback. His running dash attack was changed from the fireball to the running Yo-Yo move from Kirby Super Star. As said, he's now heavier then Fox.
Zelda was buffed. Her Lightning Kick (AKA back aerial) does insane damage and knockback now, though the "sweet spot" is smaller now. Overall seems a little faster too.
Sheik was slightly nerfed and buffed. Her movement speed is slower, but her attacks seem to have less lag.
Mr. Game and Watch seems a little faster, and his moves overall have more range and power to them. His Up B now goes higher, and he floats down using a Parachute.
Samus was nerfed massivly the most. Her Missiles and Charge beam have nowhere near the knockback they used to which are her main staples. She did get buffed up elsewhere though. She runs and attacks faster, and Screw Attack seems to go higher and has more power, and her "crappy" moves (Like her arieal Forward A) has more power and knockback.
Can't say much about Snake. He doesn't seem too fast, but his Up B has good recovery. He's generally a sneaky character (go figure), able to plant mines around the stage at will.
Ice Climbers have been buffed somewhat. Nana is no longer as stupid as before, and both Ice Climbers can grab the same edge, so they won't edgehog one another. Their Smashes took a nerf though, no longer as godly as before.
No noticable changes with Donkey Kong. Someone mentioned his standing moves have a little more range, but he's not much different then before.
No major changes with Yoshi either, save for his second jump has more range to it, and his Egg Throw move propels him upward ever so slightly.
EDIT:
Ah geez, I forgot Ganny! Ganondorf was buffed a bunch. Though he's still a slowpoke (a constipated ones, if the video show anything), his moves are just as nasty. He's still somewhat a clone of Captain Falcon, with some slightly different moves. His Standing B (Wizard Punch?) comes out MUCH faster now, and both the Punch and the Wizard Kick have different animations. Instead of copying CF's Raptor Boost, Ganondorf now grabs the opponent and has them explode in his hand. Hiw Up-B has a slight change to it; if he misses with the grapple, he throws out an upward punch.
Edit #2:
Ness too >_<. Where Lucas excels in multiple hits, Ness excels in single strong hits. Ness's Yo-Yo has been buffed, making it actually useful. Haven't heard much else on either of them.