While reading some extended and not-so-short answers with graphs and codes I came to a common forum-like idea of hiding/showing an additional though probably useful content in the body of answers or questions. I would like to hear your opinion on the inclusion of such feature in the list of edit commands.
I think this is a great idea.
That feature is already used in the "click to expand/collapse this section" organization of all the FAQs. Being able to mark up a reply to do the same thing could really streamline all the SE sites. (I would love to collapse large blocks of code when reading through some of the lengthier replies, for instance.)
In fact, one approach could be to make some existing forms of marked up text automatically (and retroactively) collapsible. This could include code blocks, quotations, images, and (maybe) lines of TeX. It might even extend to blocks that are delimited by heading markup (even though that has rarely been used so far).