3. Only Choose Dot-Com Available Domains
If you're not concerned with type-in traffic, branding or name recognition, you don't need to worry about this one. However, if you're at all serious about building a successful website over the long-term, you should be worried about all of these elements, and while directing traffic to a .net or .org (as SEOmoz does) is fine, owning and 301'ing the .com is critical. With the exception of the very tech-savvy, most people who use the web still make the automatic assumption that .com is all that's out there - don't make the mistake of locking out or losing traffic to these folks.
4. Make it Easy to Type
If a domain name requires considerable attention to type correctly, due to spelling, length or the use of un-memorable words or sounds, you've lost a good portion of your branding and marketing value. I've even heard usability folks toute the value of having the letters include easy-to-type letters (which I interpret as avoiding "q," "z," "x," "c," and "p").