The are some common sense basics...
1. The domain is easy to remember and/or recognizable
2. Extension ... a .com is usually worth more than a .biz for instance
3. Size ... ccc.com will usually be more valuable than CorporateCratingComponents.com. Although that company will probably own the latter, they'd be hot to own the former.
What little bit of dabbling I've done in this over the years has really shown me that appraisal can be a nice tool, (especially once I've found an interested buyer) but it is an exceptionally subjective market. One domain I purchased I was sure was going to make me rich and two years later i'm still sitting on it... another i purchased for actual development, not thinking to turn over the domain, and I received a very nice unsolicited offer that I ran with.
That's why I keep my day job as a site designer, and play the domain market like the stock market. So I'm sure some of the pros here have even more and better information beyond what I listed above. All you got from me was two cents worth.
Good luck!
Vivvy