I see a lot of commentary that compares Joe Wilson's "You Lie!" outburst with the ruckus that often happens in the House of Commons. But one thing you are not allowed to shout in the Commons is that another speaker is a liar. A lot of circumlocutions evolved to bypass this - "terminological inexactitude" is my favorite (Churchill, of course) - but the ban is for a reason. Once the opposition starts yelling "You lie!" they have essentially abandoned the deliberative process, by questioning the good faith of a speaker. Without an assumption of good faith or a factual rebuttal, just calling someone a liar abolishes the integrity of the debating process. It ends a conversation. And parliament is about conversation. From the rule book (PDF):

Language and expressions used in the Chamber must conform to a number of rules. Erskine May states "good temper and moderation are the characteristics of parliamentary language". Objection has been taken both to individual words and to sentences and constructions ? in the case of the former, to insulting, coarse, or abusive language (particularly as applied to other Members); and of the latter, to charges of lying or being drunk and misrepresentation of the words of another. Among the words to which Speakers have objected over the years have been blackguard, coward, git, guttersnipe, hooligan, rat, swine, stoolpigeon and traitor.

The context in which a word is used is, of course, very important. The Speaker will direct a Member who has used an unparliamentary word or phrase to withdraw it. 

Members sometimes use considerable ingenuity to circumvent these rules (as when, for instance, Winston Churchill substituted the phrase "terminological inexactitude" for "lie") but they must be careful to obey the Speaker's directions, as a Member who refuses to retract an offending expression may be named or required to withdraw from the Chamber.

And that, to me, was the import of last night. One side was engaged in a civil conversation; the other was engaged in an uncivil protest. The Congress became an unruly town hall meeting - because conservatives made it one.