I'm drawing from all of the new material that has been released so it's from the first trailer, the megaman trailer, the wii fit trailer, the nintendo direct developer interview trailer, and screen shots.
To me it seems like the fighting is a little faster though the jumping seems similar to Brawl.
Bowser has been tweaked his dash attack, f-a, and what seems to be n-a are all different plus he seems to move quicker. Hopefully this is a trend that more characters are going to see new tweaks such as Gannondorf the mother ****ing main villain of the zelda series needs his own move set!
The ring looks like a boxing ring I would say that's a pretty good nod towards Little Mac being in it. Not a definite but certainly a decent nod.
Samus seems to have been tweaked as well she has some sort of new kick that I don't recognize as a normal part of her move set and final smashes are back as well it seem and so is the zero lazer but there's no inclination if this will result in ZSS like in brawl.
It looks like Pit's up B has been tweaked as well he seems to take flight much quicker in the trailer than he does in brawl plus he's got new moves but they've been talked about officially so no point in restating them here.
There's some other stuff I noticed as well but I got to get going I might update this more later.