I think the ability for basic users to edit/modify should not be allowed after a certain time or X number of responses to the post.
For instance, in the high school forum there are 100 replies to the proving an acid is dibasic thread, and then the original poster deleted he question.
Or I see questions asked all the time, and people get feed back, but go back and fix the original post. Making the response and post no-longer beneficial for anybody else, as they can not see the 'history' of the problems, wrong assumptions /calculations, etc etc.
Plus, it makes us people helping look like idiots, too!