1. If the name applies to them.
EXAMPLE) My name is Matt. I just got MattsProjects.com and MattsProjects.net. To another person named "Matt", these are valuable names. To somebody named "Joe", these are worthless.
2. If the name's subject is in demand.
EXAMPLE) Web Hosting is worth lots, and a very profitable business, assuming you do it correctly. So,
www.WebHosting.com is worth a lot of money.
3. How the name fits the subject.
EXAMPLE)
www.GimmeHosting.com I just am about to sell to somebody. It's a demanding market, yet "Gimme" sounds unprofessional and somewhat tacky. That was my mistake. (although the buyer is able to put it to good use.)
4. Length
EXAMPLE)
www.msn.com, ignoring its popularity and website content, is probably worth low-mid $xxxx (probably near $8,000 if not much more).
www.abcdefghijklmnopqrstuvwxyz.com (which is a real domain), is (ignore website content and popularity) worth less than regfee.
Hope this helps.
More can be found in the Appraisal Guide, in the Domain Name Discussion board.