I played around with importing custom textures and if you take care to use photoshop with 256 colors, it looks really sharp and nice. The colors aren't nearly as drab and washed out. Really sharp stuff like lettering gets a bit "smeared" but all in all it looks like a different game.
I'm with BliZz on this. If someone took their time, they could make a pretty awesome set of textures to be used. You'd end up with an incredible download size, but that's not really an issue now with everyone on broadband. Guys I remember when I first started playing UT when it dropped it was totally acceptable to leave your guy in-game overnight to 56K all the maps on a new server. People had no other way to get them. Personal websites weren't easy to do and free online storage for downloads didn't exist.
Check this out too:
http://hermskii.com/Forum/viewtopic.php?t=987
Herm was working on making the extra tex pack usable. He's got it done but he was hammered by Ike and is out of commission for now. The textures look incredible though.
BliZz I looked at the monster skins, very nice. It wouldn't be hard to mod those to replace the stock ones on a server. Won't help a mylevel skinned one but default would work.
@nath: I understand your point, but can guarantee you if you create a more immersive environment, you *will* notice it in the game. Doing the monsters is only the start.