1up wrote an intersting hands-on preview of Starcraft: Ghost which they played at BlizzCon. There are many interesting things about playable zerg and gaming dynamics in Multiplayer.
As noted, from our playtime, the Zerg feel a bit more fun than the Terrans—probably because the Terrans are more straightforward (seemingly to ease new players in), and also because the Zerg are so damn fast. It’s a bit difficult to shoot a zergling enemy because they zip around so fast, and when you’re a zergling, it’s relatively easy to run up to someone and just start tearing into them. Hydralisks don’t move as fast as the zerglings, but spitting poison quills (which mess up Terran vision) is pretty cool, and if you score enough kills, you can run’n’gun by playing as the infected marine. Finally, the mutalisk is fun to play, but is also an “advanced” character since it can fly, and there’s a bit of a learning curve to flying (you push in the analog sticks to control ascent and descent).