It's the attention to detail, that gets me!
The lighting effects are amazing, and the sound? OMG! Play with a good set of headphones, and you'll be sucked into the UT3 world... My heart always races, and I get just a wee bit more nervous when I can hear a DarkWalker, but not know exactly where it is, until its too late, lol! Amazing job with sound! Did I say that already?? Graphic detail? Its the small nuances, like the enviroment reacting to the rotor wind from a Manta, or how your vision is blurred when you are under water! NIce touch!
Playbility? It took a little bit for me to get used to the way the pixels are laid out. Taking and enemy down with the Shock Rifle or Enforcer takes a litle more precision, but it doesnt take away from an overall fantastic game.
If you have the horsies to play it, get it! Im running a DualCore 3.4 Pentium D, with 3GB Ram, and an 8600GT. My res is 1024X768 with high detail, and the game is beautiful! Yes I know the res is not as high as the card can handle, but I usually play all my FPS at level.. Anything, higher and it becomes a problem with my vision, which is not due to hardware :-P!