Civility is more than etiquette. Etiquette is just maintaining social conventions as required by tradition or authority.
Civility in the political context is the acknowledgment that our opponents may have a valid contribution or point to make. It's the reflection of an agreement that campaigns are not purely cut-throat competitions to be won by any means available. It's the commitment to the inherent value of the democratic process and the rule of law. It's a statement that what matters in campaigns includes what their results permit us to do through government afterward.
Absence of civility in political debate makes accomplishing anything for the public far more difficult once the campaigns are over. Instead, the tone of governing debate just stays hostile, with the losers doing everything they can to undermine the ability of the winners to accomplish anything positive. They don't want to see that, because it will make it tougher for them to attack in the next campaign. Lies and misrepresentations just keep piling up. Think today's Congress. Think the health care reform debate.
I don't mind if you label one of my ideas or proposals the dumbest thing since New Coke (so long as you explain why). When you think I'm just trying to dampen issue debate, go ahead and say so--as some have--and I'll engage on the merits of that accusation. All fair.
What I've tried to do in debate here is to discourage what I consider to be unnecessary levels of personal vitriol. Vitriol is not necessary to making most legitimate points; it rarely persuades thoughtful readers; and it's not productive of long-term constructive results. It only helps those who believe that "government is the problem", by making it tougher for us to get anything positive done through governing.
A hat tip here to Jerimee for the original debate about the meaning of "civility" which produced this post.