Top Ten Mistakes in Web Design

1. Bad Search


2. PDF Files for Online Reading


3. Not Changing the Color of Visited Links


4. Non-Scannable Text


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

Read more at Jakob Nielsen's Alertbox.
[Source: Jakob Nielsen's Alertbox]