So I spent some time with this baby at Blizzcon and attended some panels about its single player.
First, the single player. It looks like Blizz is taking a more non-linear approach to the single player. They demoed a bit of the Terran campaign and it seems that Jim Raynor is the focus. You can interact with crew and you have dialogue options much like KOTOR. You can also pick and choose what missions you want kinda like Command and Conquer but on a much, much wider scale. They also have a new system in place where you can take optional missions to make money and upgrade your units.
As far as the story goes, in the cutscene that we watched Jimy Raynor is walking in his ship alone and suddenly a wounded Zeratul appears before him. He says to Raynor something like "The Xel'Naga....are coming".
As far as the new units go I was really, really impressed. They Protoss have this badass new flying unit called the Mothership that drew cheers everytime it came on screen. This thing was owning Terran Battlecruisers left and right. Then, when it got surrounded, it opened up this black hole that sucked in like 3 Battlecruisers. It was pretty awesome to watch.
I got to spend an hour playing it and I loved it. The game just felt very fluid and just felt like Starcraft. It was very, very fun and best of all it really looked like it was almost done. The single player cutscenes we saw all had voicework and the gameplay looked very polished. I'm going to hole myself in my room for several days after this is released.