"When disagreeing, please reply to the argument instead of calling names. 'That is idiotic; 1 + 1 is 2, not 3' can be shortened to '1 + 1 is 2, not 3."
Because we've recently been getting a series of low quality submissions which obviously drive low quality discussion, but for some reason it's only the commenters who get reprimanded, as if the submissions weren't ruining the mood one by one.
(And to clarify, I haven't been warned, I'm not writing this out of spite.)