I have ut3 on steam too, but honestly, I see it as a completely different type of game.
q3a is a fastpaced fps played in a small map, whereas ut3 is usually played in bigger maps, with vehicles and shitload of different weapons and shit. I enjoy playing in coop though.
Another reason why I like QL better is that old engines like quake and goldsrc have very simple physic and movement systems that are actually fun to play. (strafing, bhoping, etc). Newer engines though limit you so much to make it feel more "realistic", like, you don't even get to actually go up ladders or even fucking jump for yourself anymore in some of them.
CS was known for having shitload of exploitable bugs which lead to KZ and SURF. Many of them were actually pretty underground and unknown to most, such as the "jump bug" which consists of unducking and jumping 4units before touching the ground, and you get no fall damage (it gets harder if you were falling from higher since you have a higher velocity and it's insanely hard to time it).
EDIT: Just as I finish typing this, some dude sends me this:
http://www.youtube.com/watch?v=7z4QJoMVCPQ&hd=1