I picked the "I play only Aliens/Humans." Because I will nearly always play aliens. I will, occasionally, play humans, but I don't have nearly as much fun.
Quake (any of them, really) has always been about movement. Once you get the mouse in your hand, aiming is pretty easy--force yourself to play railgun only in Q3, and you'll get pretty scary, pretty quickly. The harder thing is figuring out how to get the heck out of the way.
I'm also a fan of speed running (meaning, I'm no good at it, but I think those guys are Teh 31337).
Anyway, what this boils down to is that the humans run slow compared to standard Q3A, and aliens generally move on par speed wise, and have a big bag of tricks, most of which involve movement. I like figuring out how to get to difficult spots on a map, and the aliens open up every little nook on the map, if it's accessible at all, one of the aliens can get to it. I also like figuring out weird shortcuts on maps.
Again, I don't play aliens 100%--there is wisdom in knowing one's enemy. (It's still a good bet that I'll join the aliens if you see pop onto a server.)
Jeff