I read (other than work related technical stuff) for recreation. When a web site I need to read to buy the book doesn't work right, I know I won't enjoy the book. Regardless of why it doesn't work right, be it poor design (can't find a buy link), use of flash (or worse, animated GIFs - they never work right) or other bling, or having to dig through mounds of unrelated rants to find the info I want. It's just now worth it.
But mostly, it's poor design choices. Color combinations that make my brain bleed, flash, lack of links to what I'm looking for, and so on. Sound actually doesn't bother me, because it always keep my speakers turned off, all the time, except when I know there's a reason to have it on.