Well what I've noticed from their previews and the new unit description is that most of the units cannot be used to their full potential when they are in a mass group. Like in Brood War you could do a Mass of hydras and do just fine unless someone Storms/Reavers/BCS/Guards you, well you get the idea.
You could just go with one unit and do pretty darn good( Unless your going against me hehe )
But in SCII it doesn't seem like you can get away with that.
i hate it when ppl go a whole variety of people i just go all out on one thing but corsairs and carriers kick BC's but or mutas i dont know how i feel bout this
Hmmm. On the one hand, it makes you think your strategies through more. On the other hand, it requires much, much, much more micromanaging, and if it's as hard to micromanage individual units as it was in SC, it's going to be a pain.