God help me, I'm about to say that something that Facebook has done (recently, btw) is a good idea.
I'd like to be able to see how a Forum post has been edited since it was originally posted. Although I imagine most of the time people edit to clarify their original intent or to fix grammar and spelling, I know that sometimes they do it in ways that shift the terms of a debate and undercut the responses and arguments of others.
I'd really like to be able to see the edit history of posts.
ETA: Full disclosure—I edited this post to clarify my point and to remove an implication that the linked post changed its title IN ORDER TO undermine responses. I don't know that was the intent and didn't intend myself to say that I did, so I reworded.