I've seen quite a few questions that ask how to do something with some software that happens to be able to run on Ubuntu, but could equally be answered on any Linux related stackoverflow site. They have nothing in particular to do with Ubuntu directly.
When are such questions off-topic? When should they be redirected to SuperUser or serverfault or such?
For instance, this one got closed: https://askubuntu.com/questions/100153/how-to-install-mac-os-x-lion-10-7-on-virtualbox-4-1
But this one is apparently ok? What is IOMMU and will it improve my VM performance?
I bring this up because I was considering flagging the following but wasn't sure: https://askubuntu.com/questions/100235/bind-9-on-ubuntu-with-special-functionalities
I figured to myself that a simple rule of thumb might be "if this question could be asked on AskSUSE with only s/Ubuntu/SUSE/g done to it, it's likely offtopic". Is that about right?