Generic, commercial, definitive, popular.
(A) Generic = a word/phrase/expression that is used in every-day English in conjunction with the niche in question. For example, if the niche is double glazing, generic domains might be: glazing.co.uk, doubleglazing.co.uk, windowpane(s).co.uk, pane(s).co.uk, glass.co.uk, etc.
[The opposite of generic, in domain terms, is a coined/brandable expression. In the above example it would be something like A1glazing.co.uk or Fleurble.co.uk - might perhaps be of vague interest to someone, somewhere, sometime, but it doesn't immediately ring any bells and isn't a typical English expression (or even English at all, in some cases)]
(B) Commercial = a word/phrase/expression from a niche that has commercial value i.e. with companIES (note the plural) battling to dominate the niche. "antidisestablishmentarianism" is a generic term, in the sense that it is a dictionary word, but it has absolutely ZERO commercial value. Sometimes non-commercial generics can still have value if they're short/memorable e.g. aardvark.co.uk (has the added benefit of coming first or near the top of any alphabetised list)
(C) Definitive = niche-defining. If the first expression that 9 out of 10 people consistently think of when they think of a niche exactly matches your domain, that's pretty dominant. For example, if everyone thinking about buying double glazing instinctively searches for "double glazing" first, then doubleglazing.co.uk would be the definitive domain for that little niche.
(D) Popular = size of the niche. If you have the best term in the world for a product that only 50 people are interested in, your market is very very very limited. On the other hand, there are very small but competitive niches, e.g. private jets (only a few hundred are sold every year, but they are worth a TON of money).
Putting it all together...
- Best domains have ALL the elements (A)-(D) present.
- Ok domains can suffer a bit on (B)-(D), but (A) must be present i.e. genericness is the definitive base criterion, and the other criteria are value drivers.
- Domains that fail (A) (i.e. brandables) can still make a lot of money, but it's the domain registration equivalent of buying a lottery ticket - you often hear about the (relatively) infrequent successes, but there are tens of thousands of unsold - and most likely unsellable - brandables hiding behind every success.