I'm still waiting for an invite for the Mac beta. I've played two games on my buddy's PC though as Zerg and Terran. I played one against the comp as Zerg and one online as Terran. The comp was too easy which inflated my self-confidence. I played as Terran online and got wasted in under 20 minutes.
It's very much like the original but IMHO that's a good thing. There are still balance issues though, hydra's are a bit overpowered (ask me how I know). Against the comp as Zerg, I found it easy to just mass hydra's and overwhelm my opponent. The same thing happened to me while I was Terran.
Firebats got replaced with Hellions, which are like Vultures with flamethrowers. I wasn't able to make it far up the tech tree while playing Terran so, my experiences are limited.
Overall, the game looks REALLY good. Think Starcraft with the WC3 engine but even more polished. Use of high ground is really important too. The differences in the first game between the races is even more stark now. More than ever you need to mix Terran units to make them even more powerful. In SC1, I used to mix Vultures and Tanks and easily get by, now, I think it's going to take better micro-managing skills to really shine with Terran.
I've since reinstalled Brood War and started playing online again. I'm on the East playing as CheesyTotz if anyone is down for a game.