I am in the process of switching over a community from mailing lists to discourse, and am coming across the issue that mailing list users like to send one word (or even one letter) emails to their mailing lists, and I don’t want to prevent them from doing that since I want the transition to be as seamless as possible. Some of the mailing lists are not heavy on discussion but rather are more about coordination of meetings, sending birthday greetings, informing about office stuff eg there’s snacks in the kitchen.
Meanwhile we are switching to discourse in large part because we want to start encouraging the civilized discussion that discourse was created to facilitate. 
The answer, methinks, is to provide category-specific title and text length settings. There might also be other settings that need to be reviewed for this, but these are the most troublesome on our site.