This is a question spawned from comments to my overly broad opinion piece on the 90 day review. I started responding there before realizing that this really deserves its own discussion thread.
The relevant bits:
This site will always have trouble with what is or is not on-topic but I don't think the grab bag style is going to work in the long run. If a question is about math, send it over to Math.SE. If the question is about the English language, send it over there. The same goes for Physics, Biology, Cooking, Cycling. Don't try to syphon off topics from other SE sites. But even more than this, don't accept questions just because there isn't an SE for it yet. If an SE did exist and you would migrate it, the question should probably be off-topic. This leaves the question about what is on-topic and I think the answer to that should be Skepticism. You can have questions about all of the above topics that would be on-topic for Skeptics. The point isn't to send everything over to the other SE sites. The point is to ask, "Why does this belong here and not there?"
To form this as a launching point for further discussion, what should our pattern of acceptance be when dealing with questions that other SE sites can handle? At what point does the fine line cross from on-topic here, off-topic there and the opposite?
Presumably there are some examples of on-topic in both places. I am not trying to suggest an assumption across all possible questions. But in general, what should we do? More of my thoughts will be in the answers.