After watching the entire 21 minute 'live' introduction, I can summarize the main cool points:
-Everything is really 3d!
-Some units can jump or walk between levels of terrain.
-They appear to be introducing a new class of unit that you can only build one of and own everything. The new Protoss Mothership is an example of this. It's specialties are slowing time and creating mini- black holes that can suck battlecruisers in. Each race will probably have something similar...
-The game is more about properly balancing your units than ever before. For example they showed off a Protoss unit that's almost invincible to heavy Siege Tank fire but quickly dies from light Marine fire. There are units that kill large, slow units like nothing but likewise die from many small units. So I'm guessing you're really going to need to have a large variety of units in order to win.
All in all I really can't wait to play this!
And you can watch the entire video here: GameVideos link