Hey, It's That Sound!: If you're a fan of the Unreal games, you'll notice that there're several event/weapon/explosion/ambient sounds carried from older Unreal games. Hell, you can even hear some sounds from the first Unreal game in some stages of Unreal Tournament III.
Promoted Fanboy: The first Unreal Tournament came to the stores with around 8 mappers having worked on it, some of them gathered from the same Unreal community. The game was a success. Also, its modding and mapping community grew very well. By the time Unreal Tournament 2004 came out, the quantity of mappers grew up to almost 20. The most played map of the game, Rankin, which also made its way onto the demos, was made by the community mapper Hourences. He also made the most played Onslaught map of the game, Torlan, also in the demos as well. Both maps were made for that game in particular. Hourences is now working on the RPG The Chronicles Of Spellborn, and has released two books called "The Hows and Whys of Level Design".
Due to this, Epic Games tried to do UT3 more "modding friendly", and the success it has is still up for debate.
In an interesting bit, before the launch of Unreal Tournament III, a short contest was done in the Epic Forums, to give a name to a map. The winner was keyrat, who was credited in the game.
In a lesser note, Vehicle Capture The Flag is a standard game mode in UT2004, however as no maps shipped with the game that use the VCTF- prefix (and thus are recognised by the game as Vehicle CTF maps), the game mode is hidden. Only by downloading and installing user-made maps for the game mode can you play it. By UT3, Vehicle Capture the Flag lost it's identifying prefix, meaning some regular Capture the Flag maps would have vehicles to use, and others wouldn't.