Aliens have always been a staple in scifi games, so the protoss, while cool, are pretty much the basic (more civilized elf/vulcan but reptilian and clad in gold with a tiny Egyptian flare) The terrans are... well humans, so while the siege tank, viking and storyline are unique, "humans in the future" is even mode widely stereotyped than the aliens. The Zerg fall under the category of zombie AND alien. They are the "pure evil" sort of enemy BUT I must say, The zerg are the most UNIQUE.
i have never heard of any other seiries having a race of "many races" all different animals with similar characteristics and of a single mindset (evil) The fact that the buildings are alive really set it apart. Although the zerg are not my favorite race to play as, i can certainly respect them and admire the concept.