For non-premium domains: I look at market relevancy, the number of advertisers (on Google, Yahoo) under the exact search term plus the bid amounts - particularly constellations of higher bids which suggest strong competition for the search phrase by companies within a lucrative industry. Also consider OVT for general indication of search popularity. Raw Google listing results are a lesser consideration though noteworthy. Then you have things like exisiting traffic, backlinks, daily revenue (and its trend over months), page rank, and are other extensions reg'd? (and being utilized), prior sales in other extensions, etc.
For premium names: all of the above become less important down to nearly insignificant. Premium domains can be propelled into huge success and income generation that obliterate common ppc income models. Now heavy traffic will enhance inherent value but is not a prerequisite unless dealing with a reseller or pure PPC domainer (tiny % of potential end users/buyers). Premium names have instant authenticity and brand potential that defy so-called appraisal formulas.