The biggest thing that I learned in 2004 was that a website consists of three things:
Teaching this to people who know little about computers and nothing about web design is tantamount to attempting to train a cocker spaniel to land a 747.
The reason we go to a website is to find content. Anything that gets in the way of that search is a great reason to look for the information on a different website. I believe that's the best tack to take when I try to explain why a Flash-introed site is a Bad Thing for a dry, corporate client.