Enhanced psionic powers grant them protective, regenerating shields and strange, terrible effects akin to magic. Protoss units are costly in terms of resources, but tough as nails. Buildings have to be within the range of energy transmitting Pylons, lest they be rendered useless. The Zerg have evolved a purely organic society. The drones that collect crystals and gas can also mutate into living buildings.
Units improve by evolving the species to gain range, speed, or powerful new abilities.
The lowest of Zerg units are especially cheap to breed, but have little capacity for damage. Unfortunately, most people find that the Protoss overshadow the other races, but only slightly. Graphics, of course, are splendid, but we would expect no less. Animations are fluid and nicely detailed. You'd actually think you were watching the movie Aliens in some of the fights against the Zerg.