Quake 3 is all about the rooty-tooty-rocket-shooty. DeFRaG strips all that away, and we're left with pure motion. You could be a dancer. You could be a gymnast. You could spend your entire weekend hopping along the same strip of Gothic castle hallway trying to shave a millisecond off of your god damn record. It's totally up to you.
Natural Selection was the mod that pitched Aliens against Marines in one of the most challenging and compelling gameplay experiences of its day. The conflict between these two teams was at the core of the game, and the massive variety of gameplay opportunities that this conflict provided made every match unique.