If you watch the gameplay demo on youtube you will see. Basically a lot of ground units can hop from level to level. while otehrs cannot. This will cause some strange thigns for sure.
Also protoss have a new transport ship that can act as an emergency pylon. This will hep them a lot I think. I think toss lose cloaking. No more dark templars and I haven;t seen arbiter
Zerglings can now mutate into some sort of volatile rolling chemical bomb. Kidna cool.
I suspect marines will now be a medium unit as opposed to small. I base this on the fact that only 4 marines got out of each drop ship in the gameplay demo.
On a side note I don't like immortals
I think dragoons were way cooler. They should have kept em and just change the graphics so they look more like they did in the original cinematics.