Jakob Nielsen the web usability guru has written up the Top 10 usability mistakes based on his research, which I have summarised below.
1. Legibility Problems
i.e Low contrast text, small or frozen font sizes. This is by far top of the list, scoring almost twice as highly as non-standard links.
2. Non Standard Links
Links that aren't coloured differently or underlined.
No differentiation between visited and unvisited.
No explanation as to what the user will find at the end of the link
(don't just write "click here").
Don't open pages in new windows unless necessary.
3. Flash that serves no purpose
Main point is unnecessary splash screens and flash intros. Using flash to jazz up a page is a general no-no. Don't use it for fancy navigation - people want things to work how they expect.
4. Content Not Written For The Web
i.e Making it short and scannable. The content should answer users' questions and use their own language rather than your company language.
5. Bad Search
i.e Irrelevant results. People like searches to behave in the way of popular search engines google, msn and yahoo.
6. Browser Incompatibility
Enough people use alternative browsers for you to cater to all of them.
7. Forms, Unnecessary Fields.
Only include what is necessary. Don't make fields mandatory unless they really are.
8. No contact Information
Physical mailing addresses on a website are important for credibility.
9. Frozen layouts and fixed widths
This point mainly goes back to point 1 with fonts not changing in size with the resolution. Sites that also go to horizontal scrolling are also a problem.
Don't use them!