To design a successful website, here are some key factors that you should take in mind:

• Usability, Site Structure and Navigation - It should be easy to navigate and find information on your site.
  
• SEO/SEM - You should appear on search engine results to get more targeted traffic.
  
• Content & Copy - Your content should persuade and support call to actions.
• Web Design and Layout - The design of your pages should support your brand and should be pleasant to the eye.
  
• Accessibility - Your site should be accessible even for the disabled.
• Build Quality - How long does it take the site to load? Does it display the same look throughout all browsers?

I found this helpful link through Smiley Cat’s Web Design Blog. The link basically provides you with a checklist so you won’t forget the important details in designing a website.