Very annoying to users of screen readers and other adaptive technology. Authors of inaccessible webpages are usually unaware that their page is not accessible, and don't realize that simple changes could fix this. Common signs of an inaccessible webpage are:
lack of alt tags on immages, excessive use of tables, and reliance on java applets. The bobby tool at:
http://www.cast.org/bobby
is a good start to making your webpage accessible. But, its just a start!