I've noticed that there are often questions on SO that don't really belong there, because there is a dedicated site for the purpose, but probably get asked because the user hasn't found that.
For example almost every question I see about SharePoint might be better answered over on SharePoint.se than on StackOverflow. I tend to add a comment to that extent when I see them, but I wonder whether it might be possible to add a clear pop-up or message when a user adds a tag like "SharePoint" to say "Hey, did you know there is another Stack Exchange site all about this topic?" in the same way we flag up obvious and catchable problems in questions right now.
It might save a bit of admin work in terms of moving questions between sites and help users to get their answers a little faster.