3.0 here. Part of it was that I didn't like some of the rules changes, like facing and weapon sizes. Another was financial.
When the revision hoopla started, they said we'd get some revised classes, some material from various splats put into the core, and errata and various spell fixes to solve some problems. I had visions of me getting the 3.5 books and handing my players a list of changes so they could for the most part continue using their 3.0 books.
When the changes actually hit, they decided to rewrite nearly every spell. There would be no list, because a 3.0 PHB is useless at a 3.5 table. While I could spend $90 on the core books (less online), my players would have a serious problem with replacing all the books, most of which were in fine shape. We had a large group at the time, and I figured the total cost to be somewhere around $300. Too much for a revision that I only liked half of.
I've gotten some of the 3.5 suppliments, and to be honest on the suppliment side I see little problem using The Complete Warrior or Draconomicon in a 3.0 game.