Unanimity Minus Two may be the right model for most wikis, because they use it anyway. If everyone except two people agree that an article is all right, and two people keep hacking at it, including one reverting the other's changes, that is U-2. If one person was really isolated as a troll and the one who reverted them was really backed by everyone else, this would be U-1, but when you see more than one or two reverts, WITHOUT third and fourth parties sharing the burden of reverting, you KNOW this is not the case, and that it is really between two people.
Usually also in a courtroom, everyone except the plaintiff and defendant will agree that the decision is all right. The job of lawyers is usually to agree that it is fine, even when their clients don't! The defendant always wants to just be left alone, and the plaintiff would like to see a heavy sentence for a minor bylaw violation usually. So real communities know about U-2 and how to keep the two who are fighting, from polluting everything else going on.
Social software really has to rely on this idea completely. Trying to ask for full unanimous agreement means groups can't get large. Letting a clique just pick off people one by one, or letting one consensus thug hold up everything, leads to tyranny. And if decisions with only the support of U-3 can go ahead over the objections of three, then the three people who are left out can probably conspire successfully in various ways that aren't obvious to everyone else, and eventually kill the system off.