Article by web usability guru, mister UseIt, Jakob Nielsen. Years ago, I bought his book Designing Web Usability: The Practice of Simplicity, which I've referenced many times.

Read the article for an explanation of each of Nielsen's top 10 mistakes in Web design:

1. Bad Search
2. PDF Files for Online Reading (I hate getting a PDF file. Most of the time, the content could have been an HTML page.)
3. Not Changing the Color of Visited Links
4. Non-Scannable Text Write for online not print.
5. Fixed Font Size
6. Page Titles With Low Search Engine Visibility
7. Anything That Looks Like an Advertisement
8. Violating Design Conventions
9. Opening New Browser Windows
10. Not Answering Users' Questions