Two improvements:

1. Make it a checkbox people have to check. Just to force people to take one minor action, and read it.

2. Rotate the text with a variety of different statements, just to mix it up:

"Are you being nice?" "Does this contribute to the conversation?" "Following the golden rule here?"

and most importantly:

"Will this get upvoted?"

:P

It's a low tech solution which focuses on fostering community values.