Sounds like a swell idea. The post is still there but marked negatively, unreadable by default but at the click of the mouse.
Of course, the question of "what qualifies" is perfectly valid. The solution might be as simple as: democracy doesn't really work in these situations, so go forward with the dictatorship. Set up a moderator - or a group of moderators - in whom you *trust* to do a good job and leave it up to them. It's not easy to put all control of such matters into a single person or groups of people, but neither is it good to try to please Greeks and Trojans.
But of course, if posters could "vote" for that, as you daydream about, it would be even better.
Was the latter group against deletion or did they actually think a "fuck you" in meta-200 pixel high font and a direct death threat is not offensive? I can't believe anyone would think it would be ok, but I can see people not wanting any posts deleted (especially since in comparison no-one can delete Usenet posts once they're sent.)
In my case, and I think in most people's case, I think it was extremely offensive, a personal attack, and way over the line and out of context. But it was there, it was said. If it's removed, the author doesn't have to take the consequences anymore, and I can't be fine with that. Also, people might think they can say whatever they like, because if it gets them in trouble, it'll be removed. I don't like that. Finally, deleting information really goes against my grain, especially in forums, in which a curious sort of social interaction takes place - long conversations over long periods of time. I want to see every bit of that conversation, as long as I'm taking part of it. Even the bits I don't like. After all, we're talking about an extreme case now. Some other times, it isn't that extreme, and it's harder to see whether, for instance, someone is being racist, sarcastic, or just making some sort of point.