[No-Man]

Blizzard is working off of very old prototypes. Essentially, there are two kinds of aliens that invade earth. There are the biological, perfect hunter beings (like the Zerg) and then there are the technologically superior beings (like the protoss). If we're not being invaded, then the aliens come one at a time, like in E.T. and are usually very cute and amusing in their misunderstandings of our culture.

Let's just throw out some examples of the three kinds of aliens, shall we?

Protoss = Predator = The Covenant = Skinnies
Zerg = Alien = The Flood = Bugs
Terran = Human = Masterchief and Co. = Humans

It's been done before and it will be done again. It's not a bad thing, but you can't really say that anyone is really stealing from anybody else.