Wew... This is a big topic. I will list a few criteria, but there are many other ones as well that others can add.
1) extension
2) domain keywords
3) length of domain
4) traffic to domain
5) page rank
6) alexa ranking
7) domain revenue
8) marketability/brandability (i.e. developable?)
9) how long domain has been regged for
10) search engine links (e.g. google, msn, yahoo)