We often see new users make offtopic, low quality posts, with broken English and cries for help. These questions look significantly different from most on SO.
When I worked in publishing, we would frequently receive documents that were incorrectly formatted. This was a major problem and we wasted a lot of time sending emails back and forth.
I made a small JavaScript quiz asking our authors about the formatting directions and the number of emails drastically dropped. I felt this was much nicer than simply rejecting them, additionally providing us a chance to make the directions more clear.
It is slightly unconventional but, would showing new users a quiz where they have to identify good and bad posts help to improve the quality of SO questions?
Edit (3/8/2017)
Recently a Norwegian website got some media attention for implementing this kind of test.