An OpEd, or opinion/editorial, is an article in which the author states his opinion. It typically consists of a position, some evidence, and a call to action. I thought this stood for "open editorial", and was the section of the paper where reader comments are published. Well, OpEds usually are published in their own part of the paper, but I thought that on-staff people could write them too...
I believe Op-Ed comes from the fact that they are/were usually placed OPposite the EDitorial page. The editorial page contained the opinions of the paper's editorial staff; the op-eds, on the facing page, gave the signed opinion pieces of other authors.
The responses can be of only a few kinds: contributory agreement, constructive disagreement, thoughtful consideration (which consists of distinguishing different contexts and then agreeing in some of them and disagreeing in others), and outright FlameWars.
On wiki, sometimes an OpEd will prompt the creation of whole separate pages for the opposing point of view.
When there is broad consensus, OpEds can be refactored into PatternMode or DocumentMode, but often they remain in ThreadMode, and die when they scroll off the RecentChanges page.
One of the hard things about writing an OpEd is ChoosingWikiNames.
Another hard thing is trying to change people's minds. You can't do it. MyMindIsMadeUp. :-)