As mentioned above, being passionate (or at least interested) in the topic of the site is often very helpful. It is easy to "let a site go" if you are not interested in the topic. I would also agree that keyword names are easiest to develop, especially those with a great number of exact monthly searches. Of course, there are plenty of successful names that have nothing to do with their subject matter that have great success. If longevity is a factor I would stay away from names that are trendy in nature. If you want to develop a site for the purpose of flipping it, then trends can be good.