I love UT3 for six reasons:
1) Ever since Patch 1.1 came out, UT3 became decent to my standards. It was missing one simple thing in the retail (the model height problem), but it never bothered me.
2) UT3 (and all other Unreal Engine 3 games) includes Kismet, for unexperienced coders, like myself
3) Character Selection (aka creating your own character) is pure genius imho.
Probably THE best addition to the UT series.
4) Single Player mods is long and awesome, but hard, just like the old games were, hard and long...but UT3 is unique to that matter ebcause mainly MMORPG's and RPG's usually have that kidn of gameplay, UT3 brought it to the Unreal Tournament franchise.
5) I am 100% obsessed with the Rocket Laucnher model, the Bio Rifle model and the Link Gun model.
6) The Techno styled music that was also in Unreal 1/Unreal Tournament was a good choice for UT3. The second best addition they made.
I have a small amount of gripes to the game as well, but the game is too full of win and awesome to be griped about (except some very few times). If I was griping about UT3, there would be no UT3DOM.