Well, some of the clues are: - maximum five generic meaningful words or terms, in context - smaller than 20 characters - no numbers - no hyphens - not a typographical error or "typo"
Its varied among people. But for me the premium domain is something catchy,memorable,branded,pronounceable.
It's as subjective and tougher than rating members of the opposite sex. There may be some disagreement if someone is a 5 or a 7 but when you see a premium 10 then there is usually no argument from others that she is a 10.
I always take into account the targeted keyword or keywords in the domain. Since you're going to benefit from backlinks coming into the domain over time, having at least the main keyword for the product/service/etc. in the domain has some long-term benefits for ranking.