Domain name subject matter that is something that people routinely look for online = Direct navigation traffic.
Growing and competitive market space = Pool of bidders.
High value leads = higher PPC cost and payment for sharing traffic. Generally this means generic, highly commercial, industry keyword phrases. In other words, the domain "IS" the generic, non-trademarked phrase for what people are looking for. (See past threads about geotargeted domains, i.e., City+Service.com or State+Service.com )
Demographics: Big markets are better than smaller markets = More people looking.
Given 1-4 above I sometimes manage to register domains that find favor with click traffic consumers. Domains earning domainers $10-$20+ are often the same domains that generate 10-12 sales leads, leads that may generate $5,000-20,000.00+ in revenue for the end-user consumer of the traffic. So I receive offers, from time to time, that I might not if I didn't chose the above model.
Food for thought: I've chosen to target and share the idea of targeting localized domains. Why? Because I'm reasonably confident that a) Local search is just beginning to emerge; b) Local businesses, a huge advertising market, are just beginning to wake up to the need to geotarget or localize their traffic; and, therefore c) The PPC bids for localized services are likely to grow in the coming years, both in volume and in what it costs to "get the click".
You can't compete with domain tasters on their own playing field but you can use your life or work experience and insight to make a successful move.
[edited by: Webwork at 11:10 pm (utc) on Feb. 12, 2007]