I second benevolent001's link, it's a matter of coincidence that mack just posted that a few days ago.
Another, if you haven't built your site yet, or even considered the concept, start with this question:
What problems will my web site solve for my visitors?
It doesn't matter what the problem is - boredom (youtube) buy something (ecommerce), sell something (craiglist,eBay) - if your site doesn't solve visitor's problems it will fail.
Source, and much more good advice on what NOT to do: webpagesthatsuck.com [webpagesthatsuck.com]
Most site owners get this backwards - they start with the design, the look. Visitors don't come to your site to ooh and awe at your design, they come to solve a problem. Crack this nut and it will all start to work for you.