1. Bad Search
When someone arrives at your site they will navigate through it in one of two ways, by browsing through your navigation, or by using your search function. Does your search function analyze its queries too literally? Have you tried doing a search on your own site and made a spelling mistake, entered an SKU or telephone number with or without a space in the middle of it? Most web site search engines do a terrible job of handling anything but exact matches. This will end up driving visitors away from your site as they are being told by your search function that you don't have the information to help them. You must strive to stay on top of your search function by constantly auditing what people are searching for, and then checking to see what results you are serving them. After this you will need to make your search function smarter so that it can properly handle these queries in the future.
2. PDF Files for Online Reading
PDF files slow down the users experience, can be harder to read as they are formatted for print and not screen, and they require software to open and view them. PDF documents have their place and should be used accordingly. Any documents that are to be printed,(i.e. a printable version) should be in PDF. Documents should be available first in HTML so that they can easily be read.
3. Not Changing the Color of Visited Links
Part of knowing where you are, is knowing where you have been. Knowing where you are and where you have been will help you determine where you want to go, and not go.
4. Non-Scannable Text
Web users typically have short attention spans and are not willing to wade their way through volumes of text. Many users will retreat from or leave a page when it opens containing a large block of unbroken text. Text should be skimmable and scannable, and written in an inverse pyramid style.
Break blocks of text up using subheads, bullets, bold or highlighted key phrases, short paragraphs, and small photos.
5. Fixed Font Size
Many websites today are designed so that the text is visually pleasing and blends into the esthetics of the site. Unfortunately this often means that the font is small, and not highly contrasted to the background. Many of todays web users require corrective lenses and find reading small fonts difficult. Fixing the font size to ensure a visual look can alienate a large user segment. This is also an issue of accessibility.
6. Page Titles with Low Search Engine Visibility
Page titles are tremendously important in helping search engines ranking your web pages. They are also used when users bookmark your pages and thus how easily they can find your bookmark again. As well they are used to title the browser window or tab that your web page is on, help the user find your site again when they have many tabs or windows open at the same time.
7. Anything That Looks Like an Advertisement
Banner blindness is a condition that has been cultivated over the last 10 years as users have been trained to ignore anything that looks like an advertisement. This being said it is very important not to have any of your sites important assets look like ads, otherwise you risk them being ignored.
8. Violating Design Conventions
Consistency leads to trust and comfort. If you try and buck the trends you will be forcing users to relearn how to find there way around. Trust is very important in the online virtual world. If you don't have a large brand and you try to go it alone you run the risk of being alone.
Jakob's Law of the Web User Experience states that “users spend most of their time on other websites.”
The above comments are based on Jakob Nielsen's Alertbox http://www.useit.com/alertbox/9605.html