Here are a few I consider.
1.) Google Results. The results have to be relevant.
2.) Word Tracker. It shows that there are people searching for a term.
3.) PPC Ads. It shows that there is a way to monetize the domain.
4.) Extensions taken. See what other exact matches, and similar domains are taken.
In general I like to stay away from "Brandable" names. Those names are too hard to value.
I also like markets with liquidity. LLL.com, LLLL.com, CCC.com
With those markets
1.) Letter Quality.
2.) Letter Pattern. CVCV, VCVC, AABB, ABBA, ABAA, NLL, LNN, etc.
3.) Pronounceable. Is the domain EASILY Pronounceable?
4.) Acronyms.