Imagine you get together with a friend to play a little ping-pong. Only, your friend feels that the game needs a story. While you're playing, he concocts this tale whereby he saves the world by beating you at ping-pong, with each point he scores making him a greater champion against your villainy.
That's pretty much what it's like playing the campaign mode of Unreal Tournament 3. The prerendered cutscenes tell a tale every bit as compelling as the plot of a ping-pong match. Their presence doesn't hurt the gameplay, but it has the unfortunate side effect of making the whole thing feel sort of silly.
Not everything needs a story. Trouncing your friend with a whiffle bat is a perfectly legitimate way to amuse yourself when you don't have anything better to do. But if you dress up like a knight in cardboard armor and pretend your friend is a dragon, then you're suddenly LARPing, which means you're making fools of yourselves.
I'm not saying UT3 is a bad game, I'm just saying it's not as much fun as hitting people. Which is true of most videogames.
Shamus Young is a programmer and writer by trade, videogame nitpicker by inclination. If you have the patience for more of his ramblings, they can be found at ShamusYoung.com.