Don't Ignore Users' Expectations
Many topics fall under this heading, but I'll touch on just a few here.
Consistency adds value. The look and feel of your site shouldn't change dramatically from one page to another, or people will think that they've accidentally clicked a link and gone to a different site. Confusing your visitors is never a good strategy. The other side of that coin is that having a consistent look throughout your site makes it easy for people to use your site as they go from page to page—even if they entered through a deep link rather than the home page.
Keep your tunes to yourself. Unless the user clicks a button that is clearly marked to play some music, don't play music on your site. Almost everyone hates background music, and everyone hates sites that force music on them. The obvious exception is a musician's site; even then, I think it's better to put the user in charge. Simply providing a mute button isn't enough; visitors will be more likely to hit the browser's Back button or close the window than to look for your mute button.
Don't use pop-up, fly-in, or other animated divs. Some news sites fly in a "related content" window when you scroll near the end of the current page. It's distracting, annoying, and just plain tacky to try to shove more content at a user, as shown in Figure 3. You can practically hear the site muttering desperately, "Don't click away…c'mon, don't click…." Desperation is never attractive.