For example, the Pathfinder stuff. I've tried to use it in my homebrew, but it's so integrated, so well tied together, so setting specific that I just can't use it other than as-is. And I don't want to run the entire AP, and I don't want to run my game in Golarion, as excellent as I think it is.
You realize pathfinder APs =/= All pathfinder adventures? There's a whole slew of stand alone adventures(pathfinder modlues). I have many planned for my own games with minimal fuss.