I said yesterday there was different levels of risks involved with different types of products. The same is for compatibilities issues.
- Settings - from personal experience settings can be written so that they are pretty much compatible with a broad swatch of D&D editions and retro-clones. The bag of races, monster and classes have changed very little between edition.
- Adventures - With fuller stat blocks for monsters, items, etc. It gets a little harder for adventures. My opinion is that OD&D to 2e are mostly compatible, and that the within the two categories of OD&D/D&D, AD&D 1e/2e adventures are highly compatible. So a OSRIC adventures and a C&C adventure would be highly compatible with either system. The same for a Labyrinth Lord, Sword & Wizardry adventure.
- Rule Supplements - Rule system start to matter at this point. Because of the minimalist nature of OD&D and Mentzer BASIC supplements compatibility is higher than supplements between C&C, AD&D 1e, and AD&D 2e.
For commercial work this is important to get right. Because it expands our market. The larger the market the more viable it becomes. With a larger market YOU will get more variety, better quality, and better production values (like say Color poster maps).
The best is still yet to come.