Quietus is right. Skill.
The balancing between the weapons is close to perfect and they are still completely different. As they are completely different, there is a lot of complexity to the fights. Its not like everyone would be just running around, gunning down people with a mini-gun. THAT would be boring and THAT's what gameplay has evolved from in the past few years.
Fights that arise in UT2004 aren't just mindless shooting orgies, but they are dances between different weapons - poetically speaking.
Flak vs. Link
Biorifle vs. Roket Launcher
Sniper vs. Minigun
Its not only important which weapon you have equipped, but also which one is pointed at you. Its also important to know which weapons you have in your inventory. Can you fight your way towards the enemy to be able to mow him down with Linkgun fire from close range, or do you have to expect him to have a Flak?
Then, if you take out all the fighting (!) there is still skill involved in the game. A skill called "map-control". Who gets the shield pick-ups once they spawn, who is the one to get the U-damage, and who controls the redeemer? Even if you are were experienced enough to understand the fighting part, someone whith better map-control could still kill you easily, because 100 shield are still 100 shield.
If you just start playing UT you might think that everything is the same explosive, unrealistic and colorfull junk-weapon. You run around without knowing how the game really works and all it is for you is a mindless romp.
Remember the days when you first played OFP (I assume you have)?
You ran around, wondering why you die out of nowhere. You cursed the AI for hitting you from such long distances. You thought the game was a game of luck, whether the AI targets you or not, because if they do, you are pretty much screwed.
Then you played it for a week or so and everything started to make more and more sense. Now you know that OFP is a game of the mind.
Its the same with UT2004.
Another good example are fighting games (the good ones). You start playing and all they are to you are mindless button-mashers. Any depth they might have is completely beyond you. Then you play for a few hours against someone who knows what he is doing and the games start to make more sense to you.
UT, UT200X and UT3 are unrealistic, arcadey and fast-paced. But they are games of pure skill and not half as mindless as some so-called tactical shooters, where the only "tactic" you really need is to go into rooms slowly and sideways...
If you simply dismiss them for being unrealistic, you are not doing them any justice, and you might miss some excellent games that might actually appeal to you, once you get to know them better.