Not that I always live up to it, but I think that this is great:
George Washington once drew up a list of rules of civility. Here is the first one:
“1st Every Action done in Company, ought to be with Some Sign of Respect, to those that are Present.”
I’m with the father of our country. To be gratuitously nasty in public discourse is like relieving yourself in a swimming pool. Even if nobody knows you did it, you still made the pool a dirtier place for everybody–yourself included.
On the other hand, in the Internet age, who isn’t in the room? And would we really ever want a public discourse as antiseptic as a chlorinated swimming pool, absent all risks of offense, blasphemy, and cutting humor?
Prometheus Unbound 
Sorry to be taking the analogy too far, but…
Seeing as urine is relatively harmless and sterile, it would stand to reason that nastiness isn’t inherent in a particular action itself, but in how the action is perceived by those present.
Therefore, is saying something that you know will get a rise out of people a nasty action even if it’s true?
Is dissent always uncivil?