First off, go read the jetstreaming blog for tons of insights. Given everything that Paul says there -
I'd rate the value of a library on three different scales:
1) ability to use out of context:
Tim prebble's contact mics library and his tortured piano library are just overflowing with sounds that beg to be sped up, slowed down, and otherwise manipulated beyond their starting points. This springs from the out of context mentality from recording those sounds, as well as the dense and layered content in the recordings themselves. The more flexible a library's source material is, the more useful the library is in other contexts.
2) difficulty of record:
Things that occur rarely, or in difficult to access locations are generally prized recordings. Animal libraries, weapons, electricity and explosions, and certain ambiances and weather conditions fall into this category. If I don't have access to a mountain lion I'm certainly more likely to buy a library that does.
3) quality of record:
the quality of boutique sfx libraries varies wildly. Those that are of higher recording quality are of higher use to me. Quality is more than which gear you use - its also which environment you're in, the level of variations in the sound, mic positions and technique, mastering, metadata, and other factors.
some libraries rate very highly on all three scales. some do not. those that do are more likely to generate money IMO.