An article on Smashing Magazine, lists the 10 usability mistakes that you should avoid. Its an excellent article with examples of some of the popular websites that make these mistakes.

Here’s the summary of the usability mistakes listed in the SmashingMagazine article:

1. Hidden login link - Many websites forget how important it is to have the login link as prominently placed and the “Sign up” link.
2. Content in Pop-ups – Why would you want to do that, really?
3. Dragging instead of vertical scroll – This is debatable
4. Invisible links – Ok, a lot of SEO companies do that. But really, its not required. If you have links, just let the links show!
5. Visual Noise – There is a thing called too much information.
6. Dead End – Have your QA team check everything. And if some specific instructions are required in order to use the website, make sure they are displayed clearly for the user!
7. Overlapping of content blocks – Again, make sure you check your site in all browsers and all for scripts. Its no fun to have your navigation (that opens on mouseover) stuck under the main image
8. Dynamic Navigation – Its good to have an intuitive navigation. If you’re going to introduce something new, make sure you give your users some information on how its going to work
9. Drop down menus – No fun if they disappear by the time you bring your mouse over the sub-navigation!
10. Blinking Images – Spare your users the animated ads and blinking images. They may be fun for sometime, but they are very irritating when you are trying to read the contents of the page!

You can catch a list of other very useful usability related articles on AListApart.