Just a quick summary from a page we just saw. (We see some of this in the 'sphere, so it seemed relevant here. Our own observations are the stuff in brackets.)
3) Ban Right Clicks (and it's so damn easy to work around this one. we do it on principle some times.)
4) Music (almost top of our our personal most-hated list)
5) Fixed Screen Widths
6) Slow Loading Tables (: which includes stuff like hit counters and blogrolls. :)
7) Unnecessary Questions
8) Splash Pages (This pisses us off most. So many otherwise intelligent designers just love the darn things.)
9) Grabbing the Browser
10) The Obscure Plug-in
11) The IE only page
12) The Dark Page (which includes fluorescent text on dark backgrounds and bg pictures. Gah.)
Read the entire article. Do see the comments as well. Much fun.
p.s. If you have any pet peeves, and feel up to coming back here, please do add them in the comments. We promise to send on to many many designer friends.
p.p.s. Of course you're welcome to come say nasty things about our our design. We'd do the same for you. :)