Currently, there is a 40% (updated Nov 9, 2012) rejection rate on migrations from Stack Overflow to Server Fault.
Here are some samples of rejected questions from the last few days:
This rejected/deleted question about Amazon EC2 for personal use.
The first example is from 2009 and already had an accepted answer (in addition to being bad and off-topic on SF). There was no reason to migrate it. The second two, while seemingly on-topic for SF based on the title, are borderline unintelligible in regards to the content. They contained no details, no relevant information, and amount to "Halp! Halp! Itz broked!"
I realize that there is a much larger number of users on Stack Overflow that are capable of voting to migrate and as a result, it takes 4/5 votes to migrate, while every other site is 3/5. This, unfortunately, does not appear to be an effective solution.
Questions should not be migrated based on their title, they should be judged on their content and topicality on the target site. This, obviously, is not happening as there is a 40% rejection rate on Server Fault.
What can we do to further educate Stack Overflow users about what is and is not on-topic elsewhere? Should SO have a higher reputation threshold than the other sites for vote-to-migrate privs? Should a migration require 5/5 votes instead of the current 4/5?
<shameless plug>Related to your first point on the question from 2009: meta.stackexchange.com/questions/144905/…
"If we leave them on SO, though, they will get deleted."- And if you migrate them and they don't belong, they'll still get deleted, except you've just created more work for a 2nd site.