After playing as Samus and looking around at videos, She has several changes.
1) Missiles seem to have less knockback than in Brawl, but still perform about the same amount of damage as they did in Brawl. Samus hit DK with a Super Missile during the Tree House Live, when he had over 200% damage, and the knockback was not enough to kill.
2) Samus' Screw Attack now has good knockback on the last hit, making it a viable kill move, as well as giving her more vertical distance.
3) Samus' grab is MUCH faster, you can grab someone damn near immediately if they're right in front of you. However, the Running Grab has more recovery lag than the regular grab.
4) D-Air is similar to the 64 Smash version where it comes out very quickly, but seems to have less range.
5) Her Neutral Air has changed from her sex kick to a roundhouse kick that hits opponents in front and behind her.
6) Her movement and roll speed is overall faster than in Brawl, although how fast remains to be determined whether her speed has actually been improved or it's just the game being faster overall than Brawl.
7) Samus' fast fall is much faster than it was in Brawl.
That's all of the changes I can think of off of the top of my head. Anyone who is able to add onto this would be appreciated.