Disclaimer: I am a MathOverflow moderator. I haven't had time to carefully read through all the opinions in this lengthy debate, but I thought it would be useful to add my perspective.

First, and I don't think anyone is denying this, there are valid reasons to migrate some questions from MSE to MO. Whether and when to do this is the question, and this is mostly an internal MSE matter. Whatever the MSE community is comfortable with will probably be fine on our side. We will gladly welcome on-topic questions from MSE (and elsewhere) and we will not be upset if you decide to keep some or even all interesting questions here rather than forwarding them to MO.

One thing that strikes me as undervalued in this discussion is the perspective of the original poster. The differences between MO and MSE are much more than just the "level" of questions. As I recently mentioned on CSTheory, the answers that one would get for the same question are generally very different between MO and MSE. I tend to trust the users to choose the site they are most comfortable with. It's perfectly reasonable for a user to post a very specialized question on MSE rather than on MO because they want an MSE answer and/or they don't think an MO answer would be as useful to them. In making that choice, they accept that there may be fewer users on MSE with the required expertise to answer their question and similar consequences. That's the user's choice and it's perfectly fine. It is also perfectly admissible to ask a more elementary question on MO rather than MSE for the same reason though the MO community admittedly has a much lower tolerance for this (which I personally think is a problem on MO).

To summarize, I think that there are valid reasons to migrate questions from MSE to MO, though I don't think there should be a sharp dividing line between the two sites. I also think migration criteria should be rather conservative and respectful of the original poster's intent. Finally, I am against having some questions coexisting on MO and MSE since that would effectively prevent users from making the type of choices I described above.

Disclaimer: I am a MathOverflow moderator. I haven't had time to carefully read through all the opinions in this lengthy debate, but I thought it would be useful to add my perspective.

First, and I don't think anyone is denying this, there are valid reasons to migrate some questions from MSE to MO. Whether and when to do this is the question, and this is mostly an internal MSE matter. Whatever the MSE community is comfortable with will probably be fine on our side. We will gladly welcome on-topic questions from MSE (and elsewhere) and we will not be upset if you decide to keep some or even all interesting questions here rather than forwarding them to MO.

One thing that strikes me as undervalued in this discussion is the perspective of the original poster. The differences between MO and MSE are much more than just the "level" of questions. As I recently mentioned on CSTheory, the answers that one would get for the same question are generally very different between MO and MSE. I tend to trust the users to choose the site they are most comfortable with. It's perfectly reasonable for a user to post a very specialized question on MSE rather than on MO because they want an MSE answer and/or they don't think an MO answer would be as useful to them. In making that choice, they accept that there may be fewer users on MSE with the required expertise to answer their question and similar consequences. That's the user's choice and it's perfectly fine. It is also perfectly admissible to ask a more elementary question on MO rather than MSE for the same reason though the MO community admittedly has a much lower tolerance for this (which I personally think is a problem on MO).

To summarize, I think that there are valid reasons to migrate questions from MSE to MO, though I don't think there should be a sharp dividing line between the two sites. I also think migration criteria should be rather conservative and respectful of the original poster's intent. Finally, I am against having some questions coexisting on MO and MSE since that would effectively prevent users from making the type of choices I described above.