Unfortunately like a good number of you out there, I've been stuck on the east coast and thus missed out on the bountiful splendors of E4. Instead I've been reduced to browsing through 40+ page forum threads to find out what information I can. Lucky for you, I'll be posting what spoils I can find.
For the time being, there hasn't been many videos released from the floor itself and the ones that have are rather low in quality, however here are two anyway:
Yoshi vs Peach vs Fox vs Meta-Knight at Delfino Plaza (Be sure to watch for the Golden Hammer in action at the end)
Ike vs Fox at Eldin Bridge (Short but sweet)
A vid from GamePro that I can't embed:
Meta-Knight vs Mario and Link vs Pit
(Note that after you break the smash ball your character will glow until you unleash it with your special attack button, however, if your opponent does enough damage to you, they'll be able to knock the smash ball out of you)
And now for a fact list that I've compiled from reading a bunch of demo impressions:
L-canceling (a technique that will speed up lag recovery after landing after an aerial attack by tapping L or R) is not in Brawl.
When air-dodging, you no longer get to control which direction you dodge but rather move with the momentum you were traveling. This also seems to indicate that wavedashing and wavelanding are also gone. :(
There is a slight delay after air-dodging in which you will free-fall for about a second but you will now be able to double jump, or use your third jump, after. This may also be good news for characters with a tether recovery, assuming of course that L/R + A still activates it as it did with Samus and Link in Melee. Just don't expect to be able to immediately counterattack with an aerial after you air-dodge.
Crawling is character specific and can be done so by holding diagonally down. Pikachu can but Mario can't for instance.
Dash-dancing is still possible. (Its merely a technique where you smash back and forth on the control stick and you character appears to alternate directions rapidly while still maintaining the running animation. More for mind games than combat purposes...)
Short-hopping (hitting the jump button lightly to do a shorter jump) and fastfalling (smashing down while in mid-air to fall faster) are both still in.
Link has a Dark link alternate costume. Black tunic and dark complexion with glowing red eyes. He also has a gold costume.
The general consensus is that Bowser is awesome and has a possibility of being top-tier. He also has a golden costume.
Samus has a dark suit with a red visor and the Fusion suit for alternate costumes.
Sonic's alt colors are just different shadings of blue with different colored wristbands.
Fox has a Star Wolf outfit.
Diddy has a Dixie Kong outfit, without the hair of course... GAY!
There doesn't seem to be a confirmed way to start as ZS Samus in the demo build. Doesn't rule it out for the actual game though. It's worth mentioning that when you die as ZSS you stay as ZSS when you respawn.
Samus's charged shot seems to be weakened.
You can now grab an edge even if you are facing the wrong direction.
Peach's down smash is no longer broken. That means no more uncharged smash for 40% damage if you happen to be crouching when she does it. Some say she seems heavier.
Many people say sonic is overrated and his attacks lack knockback but he still requires a bit of a learning curve.
Directional influence is more noticeable. (When you get thrown and hold a direction, your trajectory well change accordingly.)
Mario's down aerial is the Mario Tornado now and apparently you can't use it while grounded.
Some information on Ike:
-He's slower than Marth and Roy.
-His forward smash is a two-handed sword swing that leaves you slightly open afterwards.
-His down special is a counter and you supposedly counter projectiles as well.
-Side B is a chargeable rush attack.
-Neutral B is similar to Marth and Roy's and can also be charged.
-His up special is Aether where he will throw his sword into the air and attack similiar to Kirby's up B. Good for attacking aerial enemies but will leave you exposed on the ground.
-Final smash is Shinkuu Aether, which is similar to Aether but he will continuously attack your opponent at the peak of the attack and then slam them to the floor for a finishing blow. Supposedly OHKO if connected.
That's it for now. I'll add a part two if any worthy information surfaces.