1.  Using Frames - Frames have a tendency to break the modality of the website. You cannot bookmark a page and come back to it. URLs will often stop working.

2.  Overuse of the "latest" technical jargon - Don't overwhelm the viewer.  Most people are not concerned with the latest technology, but are more interested in the information being conveyed. Too many technical terms or too much activity will distract the users and drive them away from your website. 

3.  Scrolling Text, Marquees, and Constantly Running Animations - The advantage - they keep your attention. The disadvantage - they keep your attention.  After a while, these things become annoying because they constantly fight for the user's attention.  

4.  Complex URLs  - If the link is too hard to get to, most users won't go. Keep it simple, and you will keep the user's attention.

5.  A site that hard to navigate - It should be easy to find the information being searched.  Too many layers or obscure links will not help to get your information across effectively.

6.  Sizing problems - Most users have no problem scrolling up and down the page, but when a user has to scroll across to view the information, they grow  frustrated and bored with the site very quickly. Making your site too labor-intensive will decrease your website's desirability.

7.  Orphan Pages - Keep Your Links Up-To-Date. It is important to perform regular website maintenance to ensure that your site's links are accurate and up-to-date.  Browser addresses and Internet information changes regularly, so be sure that your information stays up-to-date and accurate.

8.  Outdated Information - Newsletters, deadlines and content information should be kept up-to-date to keep your new and existing users informed.

9.  Overly Long Download Times - The optimal download goal is to limit download time to 15 seconds per page. Anything more is a pain!

10.  Misspellings - Misspellings are annoying, but they do happen. As soon as you find one, correct it.  It helps to keep your site's professional image.