Well, I for one, don't think UT has done much different than other games on the market. It's pretty much Q3 with the proper allotment of gametypes. That being said, I keep playing it and I quit playing Q3 a long time ago. I think alot of it has to do with the engine, it can really produce some beautiful visuals, it runs great and if your in to modding, I don't know of a more powerful, very-well documented & supported platfor for making customizations. It really is amazing how much more there is to this game than just the game (between UnrealEd, 3rdparty scripting editors, texture utilities, etc).
I'm holding out for some of the mods to be released that are in the works. I think the mod community will be the one to deliver real gameplay innovation for UT2K3. Its a shame but that pretty much always been the case (Team Fortress, CTF for Quake, CStrike, DoD for Halflife, etc.)
ZigZang