A random thought I had, for making modules that can easily be dropped into a campaign. Make your area map no larger than 24 miles across - that is, no larger than the size of a single "large" hex, using the smallest scale in common use. And don't imply that there is unusual terrain extending beyond the boundaries of the map.
I say this because of how difficult I find it to integrate area maps into my campaign, where the area map takes several hexes, and I have to find a location with (for example) mountains on two sides, and large forests on the other two sides, without redrawing my campaign map.
Bryce talks a lot about usability at the table, but I have more trouble with usability in the campaign. For this quality, I think the old TSR modules are generally superior to OSR efforts. I can drop Hommlet into most of the nations in my campaign world - pretty much anywhere I want to start a campaign. I really struggle to find a home for most OSR efforts, although
@Malrex's modules are often an exception).