I have some ideas about certain spell conditions which could be very helpful for monks to be indicated by an hint. But I can imagine you don't want AdiButtonAuras become a second Bitten's Spellflash. So what are your guidelines for what should be indicated by an hint and what not?

In general flash is for a necessity and hint for a possibility. For a rule to be included in ABA it has to appeal to a broader audience. Other than that you should avoid hinting the whole action bar as pulling attention to too much choice defeats the purpose of hints.