There's a post over at Gothridge Manor asking whether differences between old school game systems really do matter when it comes to publishing adventure materials, monster manuals or other supplements. I commented that I think it's less a matter of actual system differences and more a matter of how much system-specific material an individual reader expects, vs. how much divergent material the reader will tolerate. I don't think I articulated this very well, so I want to elaborate here.
Let's start with something that is pretty much universal across editions: opponents have hit dice and hit points. The exact numbers may vary across editions, but these differences can be ignored. In old-school systems, you need to know hit dice to determine attack capability, so adventure modules will normally list a monster's hit dice. You *could* roll the hit points yourself, when needed. So should a publisher list hit points for individual monsters, or not?
My inclination is to leave them off. I never list hp when stocking my own dungeons, and find it kind of annoying to see long lists of hp totals in a product. But I've seen other people complain about products that don't list the hp for every creature encountered. This is a difference in our expectations: I find it extremely easy to just roll hp as needed, so listing hp seems unnecessary to me and wastes space; other people consider rolling hp for 23 goblins to be more effort than it is worth, so they object to authors who slack off and don't provide that information.
When you talk about actual rules differences, such as clerics getting a spell at 1st level in OSRIC but not in Swords & Wizardry, it's really not a question of whether it's possible to convert materials to another system, but whether a particular customer's taste is for having as much prep done as possible beforehand or for removing as much clutter as possible. Again, my preference is to not even list which spells a spell-caster has prepared except for those that matter to the caster's style. "The evil high priest will cast Silence as soon as he sees an enemy spell-caster." Other spells, I can improvise; if a particular spell seems like it would be useful, roll 5+ on a d6 to see if that spell was prepped. I don't need a whole list of prepped spells. Other people would freak if they saw an encounter written up that way. How do you know what the cleric can cast? And a subset of those people would gripe if it turns out the encounter listed too many spells, or not enough, based on the system they prefer.
It seems to me that there's no tried and true method for an OSR publisher to judge how much information needs to be included and how much can (or must) leave out. It's a matter of taste, not compatibility; you just have to guess what the typical taste of your average potential customer will be and cater to that.