Sandra Rossi’s What users hate most about Web sites outlines a few problems with website usability. While the early part of the article focuses on the misuse of Flash, it does list the main problems with most websites.
I have been to more than a few websites where the ad popped up within the websites and would not go away, and it was covering a huge chunk of the article I was trying to read. No close button was available, and sometimes the ad doesn’t even display! It would be a big window that didn’t load correctly, and it would block my view.
I especially don’t like it when I have a few links that will send me to other pages within the website, while a few open a huge PDF document. A simple “[PDF]” would have helped, but letting me know the size of the file would have been better.