I'm also going to try to sum up, because there are too many different ideas here for me to pick just one other answer to agree with.
New questions with no answers
I think everyone's on the same page with these: if they're off-topic, they should get closed or migrated. Ideally, crap just gets closed (and will be auto-deleted), but there's no way to avoid individual differences in deciding which ones to migrate, so some will sneak through.
Is there a link to the FAQ on "Closed as OT" questions? Maybe there should be to give people some idea of where or how they could ask differently.
Old questions with no answers
I'm not clear if everyone's in agreement on these. (e.g. I don't know if Wesley's answer applies.) I think old, off-topic questions with no answers should be closed in place, NOT migrated, and the auto-delete allowed to get rid of them in due time.
My reasoning is that if there's been no follow-up and no answers, it's not worth possibly wasting another site's time on it. Maybe the asker is actually lurking around, but again, if there's a link to the FAQ that shows up for "closed as OT," they'll see that and try again on a better site.
Old questions with answers
I agree with voretaq7 that the general rule for old questions should be to close them in place. 90 days seems like an acceptable age to draw the line at, anything older than that should NOT be migrated unless there's a good reason for it.
To me, "good reason" would be:
- it's clearly on-topic for the other site, and
- it either
- has good answers that would be more accessible by being on the more on-topic site
- has some decent answers and is likely get more and better ones on the other site
Old questions with bad answers
This is a special case of the last category. I'd say these should definitely be closed in place and then deleted.
If there's an accepted wrong (or just bad) answer, it would be a waste of the other site's time to give better answers that won't be able to completely push the bad one aside. Maybe if it's a great question for the other site and there's just one bad answer that's not accepted, but even then, I'd err on the side of letting it die since the OP probably won't see the good answers.