Other posters have already pointed out that the game will have 2 player offline and online co-op (the online is a great touch).
This shot reminds me of that 3D Sonic fangame that popped up around the release of Episode 1. If released now everything might look a little rough (color is still a little off) but man this looks good and is a big step up.
Things we can see in these shots:
Oh snap, the Death Egg returns.
You can see it even more clearly in one of the other screenshots, and it is indeed.
Death Egg is being rebuilt around a planet (Planet from Colors? Little Planet from Sonic CD? EARTH?)
Each level might lead into the other (we see Sonic and Tails running down a snowy mountain, and other shots show the environment they end up in)
Sonic and Tails have a combined attack ability, and we see them both 'jumping' together.
There's a strange TV Monitor showing a picture of them, maybe a tutorial system for how to use said 2 player mechanics.
I wonder if the game will force 2 player making their combined abilities key to progression, or if you'll have the ability to play solo as either character.