Smashing Magazine has posted 10 Usability Nightmares You Should Be Aware of and it’s a great read. Several things in there that all developers have probably been guilty of at one time or another. Read it and then try to remove as much of them from your sites as you can.
Smashing Magazine also provided a list of 8 Usability Checkpoints as well. Just read the article for more info on each one listed below.
1. You don’t use pop-ups.
2. You don’t change users’ window size.
3. You don’t use too small font sizes.
4. You don’t have unclear link text.
5. You don’t have dead links.
6. You have at most one animation per page.
7. You make it easy to contact you.
8. Your links open in the same window.