1. Legibility Problems
Most users complain about small font sizes or frozen font sizes. Let users control the font size
2. Non-Standard Links
Violating common expectations for how links work is a sure way to confuse and delay users, and might prevent them from being able to use your site.
3. Flash
Flash is a programming environment and should be used to offer users additional power and features that are unavailable from a static page. Flash should not be used to jazz up a page.
4. Content That's Not Written for the Web
Writing for the web means making content that is short, scannable and to the point.
5. Bad Search
Search is a fundamental component of the Web user Experience. and is getting more important every year.
6. Browser Incompatibility
don't turn away customers just because they prefer a different platform.
7. Cumbersome Forms
Forms are used too often on the Web and tend to be too big, featuring too many unnecessary questions and options
8. No Contact Information or Other Company Info
A company with no address is not one you want to give money to.
9. Frozen Layouts with Fixed Page Widths
On big monitors, websites are difficult to use if they don't resize with the window. Conversely, if users have a small window and a page doesn't use a liquid layout, it triggers insufferable horizontal scrolling.
10. Inadequate Photo Enlargement
The worst mistake is when a user clicks the "enlarge photo" button and the site simply displays the same photo.