20D is bigger
20D is heavier
20D shoots a couple more frames per second (XT shoots around 3fps, I think)
20D uses different batteries (I haven't noticed a difference in battery usage since switching from the old DRebel, which used the same batteries as the 20D)
20D costs $500 more
I think there are a few more differences, but nothing major. If the differences were important enough, I'd have a 20D. If I was a pro and had to keep up the "pro image" for customers, I'd have a 20D, and I'd have an XT as a backup. The XT is a great camera, but it doesn't have "the look" that the 20D has.

If you need "the look" of a "real" photographer, then you'll be disappointed in the Rebel XT. Otherwise, I can't think of anything to be disappointed about.