1. Playing music in the background on your Web site. This applies to any and all pages, even media pages. Most people don't expect to hear music suddenly start playing when they visit a Web site. It's usually an un welcome surprise. Usibility testing has shown time and time again, that most people will immediately close a site the moment music spontaneously begins to play.
Reasons you want to avoid this:
2. Setting videos or audio clips to "auto-start". Auto-starting videos will cause the site to take longer to load in addition to often startling the visitor, resulting in quick bounce rates.
3. Unneeded Flash banners and slideshows. Having flashy, moving content that doesn't actually have any purpose, but to have it is one of the best ways to lose your visitors quickly. A useful and professional rotating banner or slideshow transitions smoothly, slowly, and not too often. The time span between rotation will vary according to what content is being displayed and what functionality it serves, but anything faster than five seconds is way too fast.
You want visitors to find the meat of your site: the content. You do not want your low attention span visitor to be distracted by a flashy banner.
4. Using image banners when text would suffice. There is a very serious problem with using banner-like images on the Web: people instintively assume they're advertisements. We have been so bombarded with SPAMMY ads all over the Internet, that we subconsciously ignore anything that resembles them.
When you use text to convey your message, people assume that it's valid content worth scanning.
5. Poor audio or video quality. While amateur video is very popular make sure you have the best video quality possible and use it wisely.