A good example is ubuntu: If you look at the questions, it's like 80% offtopic questions that should be moved to askubuntu.com, unix.stackexchange.com or superuser.com.
Or at least there could be some warning before posting a question with such tags, telling the poster to reconsider posting the question to a more suitable stackexchange site.
EDIT: Just to elaborate what is the problem. These questions usually get few close flags, but before they can get 5, they are just buried in the influx of new questions.
The ratio of off-topic vs. closed/migrated is the actual problem.