Because of the exercise of RPG product review, I've realized several things about my preferences in source material:
- Dealbreakers - there are certain things that make the product irredeemable or just not worth my time to find out if the content is worth it; this includes layout decisions that make it a pain to read through the material, or painfully inept writing or editing. Bad art, it turns out, I can forgive.
- Consistency of tone - I don't mind informal writing of source material, nor do I mind material that's meant to evoke a certain emotion or atmosphere (even if done badly). I do mind writing that alternates between the two, especially if the informal tone tends to break the so-called fourth wall. If you're going to poke fun at the atmosphere you're trying to build, just start off informal.
- Synergistic Crunch and Fluff - I don't think there's a magic ratio of crunch to fluff; I do believe that key elements of your setting demand crunch -- even if you have to say: use the crunch in some other RPG sourcebook. I hate floating pieces of fluff that are ill-defined and yet are intimated at being an "encounterable" bit of fluff.
- Organization matters - group useful bits of info together, stick them in some place close to where they're mentioned, do it consistently and I'll be happy. Throw in some indexes and I'll be even happier.