What I really hate are sites built exclusively in Flash, with no HTML version. Especially sites which are nearly all text based and are using flash to make it a bit, well, flashy. bookmarks anyone? back button?

We have a client whose new site is like this, and it adds absolutely nothing to the design. But this is the same client whose email policy requires that all their emails be sent in HTML, with 100k worth of signature images, and a default font of pale-grey. Which I can't read as I use a background window colour of grey instead of white, because it's easier on my eyes when I'm coding for 8+ hours at a time.