1.Content that’s not written for the web i.e., non-scannable text.
2.Page titles with low search engine visibility, think key words in a combination that entices.
3.Content that looks like an advertisement or content that is an advertisement, I don’t care which books on Amazon you enjoy, they wouldn’t be listed on your site if you weren’t making affiliate money from them.
4.In-your-face advertising i.e., blocks of AdSense that push your real content below the fold/scroll.
5.Violating design conventions – see CRAP Design.
6.Opening new browser windows. Let the users decide, they will come back if your content is good.
7.Non-standard links, hyperlinks should be underlined, hyperlinks should be obvious, there’s no reason to confuse novice users. Also strikethrough is being used to indicate a visited link – this is silly and should be avoided as it can be easily confused with corrections which is what strikethrough should be use for.
8.Over using Flash, Flash requires a plug-in, don’t use it unless it actually provides functionality which you cannot gain through other means.
9.Browser incompatibility, check what your pages look like on other browsers.
10.Scrolling text, marquees, and constantly running animations.
11.Complex URLs, they are not good for search engine hits or usability. Avoid.
12.Long scrolling pages, break it up, use excerpts if you have to.
13.Poor navigation. Try and find some old content on your site. Is it easy?
14.Overly long download times. Use Photoshop or other applications to optimize images for the net. Read points 12 and 8 again.
15.Too many animations. They are hard on the eye. If you must use them, use for emphasis.
Even in an ever changing environment, such as the net, you must always consider the user experience before and above everything else.