Just got back from the NY Long Island BB. I played two 1v1's on the 3DS, and two free-for-all's on the WiiU.
Overall gameplay / speed:
Characters overall feel a touch faster on the ground, and faster at moving vertically, but slower horizontall in the air. This makes the air less "safe"; it feels more like a battle for stage-ground control. Consequently I expect less "floaty" gameplay; if you're in the air, you're a bit more committed. I think this will be great.
However, the way that aerial-gameplay was "nerfed" is by adding massive lag as a "punishment" for using air-attacks or airdodges when you land on the ground. If you airdodge into the ground, you suffer huuuge lag! But same with attacks! This makes the air much less safe, but also means that the game can look really slow if players land like this. We'll learn not to, and find ways around it. But if the lag stays this extreme, it almost makes me wish for L-cancelling, even though I think L-cancelling is dumb. I really hope the Nintendo team speeds up the gameplay, and gives less landing lag for aerials.
Sonic-specific:
Sonic can short-hop (SH) Fairs and Bairs. But if you short-hop fast-fall (SHFF) them, it's maaasssive lag. This makes SHFF Bair as sad as the flop-like-a-fish SHFF Fair we all know from Brawl. Flop-like-a-fish Fair is also in smash4. Do not aerial into the ground in this game, ever.
SDJ:
If you're charging a side-B and hit Jump, you go into a VSDJ: Vertical Spin-Dash Jump. VSDJ is amazing in smash4. I don't know if the side-B version is different than the down-B version, so I'm just talking about the side-B now. Even if you start it vertically, you have more aerial control over it than in Brawl, so a VSDJ seems to give just as much of an "arc" as the regular SDJ. Consequently, I think we can just call it SDJ now.
Here's the thing: SDJ doesn't end in the air. You can just keep bouncing SDJ's without ending the spindash! I was making smooth parabolic arcs like a full SDJ but just never ending, back+forth+back, and Sonic was just a cool blue ball. It felt a lot like repeatedly jumping around in Sonic2. I think I was just holding down B the whole time, while using X to jump as I landed each time. But when I let go on the ground, Sonic did the SD hop and zoomed forward in a spindash! Like, it keeps the charge ready to go. So you can SDJ a few times then spindash forward in an attack. Amazing!
I've said that Sonic's horizontal aerial mobility is super-slow. But not in SDJ -- you have more manoeuverability in the SDJ! You can start an SDJ in one direction but then arc it backwards instead.
If the side-B (SpinDash = SD) SDJ is different than the down-B (SpinCharge = SC) jump, then we might have to refer to them as SDJ vs. SCJ. The "aura" of SD vs. SC is so visibly different in smash4 that we'll have an easier time distinguishing them.
I don't think you can fastfall the SDJ like you can in Brawl.
Spindashes (rolling):
Except for the lack of ASC shield-cancelling, spindashes are much improved. The spindashes seem to multi-hit easier, and when you land the hits, Sonic stops the spindash and returns to neutral quickly, making it super-easy to combo out of them. They have less knockback in the air as well so it's suuuuper easy to get those follow-up attacks. Fantastic.
Secondly, the side-B trajectory is truly fast and wonderful, and edge-grabs. Love.
Unfortunately, the spindashes do remember Sonic's "jump data", and I saw casuals self-destruct because of it. Sad. That was the stupidest thing about BrawlSonic, I'm so confused why they would keep that. What I mean is if you double-jump and then spindash onto the ground, when you SDJ or roll off a platform, your double-jump is still gone. Meaning people fall off the edge and die, and get frustrated because they don't know why they couldn't jump out of the spindash.
Homing-Attack:
Definitely improved, but it's probably still near-useless unless you can combo into it. If it doesn't lock-on, you still get the down-angle bounce-off-the-ground effect. If the HA lands a hit, there was one time when I was able to HA again reeeally quickly after, the way you can in a modern Sonic game. So I'm hoping that HA has very low ending lag if it hits, but I mistimed it the second time I tried to confirm that.