10 Usability Nightmares You Should Be Aware Of (via 456). Usability, in a nutshell, consists of:

• a clear, self-explanatory navigation,
• precise text-presentation,
• search functionality and
• visible and thought-out site structure.

The 10 gotchas, as mentioned in the article, are as follows:

1. Hidden log-in link
2. Pop-ups for content presentation
3. Dragging instead of vertical navigation
4. Invisible links
5. Visual noise
6. Dead end
7. Content blocks layering upon each other
8. Dynamic navigation
9. Drop-Down Menus
10. Blinking images

Roger goes on to add some good ones to the list:

Here are a few more problems that are common and make many websites harder and more annoying to use than they have to be:

• Low contrast text
• Small text that can’t be resized
• Small text that can be resized, but doing so causes content to disappear
• Preventing or breaking normal browser functionality like mouse-wheel scrolling, printing, and bookmarking
• Over-designed forms