It would be handy to have some kind of "Zen mode" here at Code Review.
Think about it, you develop code in a maximized IDE or side-by-side with the result or the specs (or both). I think Code Review needs this kind of feature. I even find myself scrolling up, read the post again, scroll down, edit, repeat. Very tedious, especially if you're handling 2 scrollable areas.
It could have a left pane containing the original post for handy reading. A right pane containing a "Zen mode" editor, much like GitHub. It could optionally (discussed in the next section) need a third pane for the preview, like JSBin.
In addition, @skiwi mentioned in his post that the answer field would be best if it was closer to the edit area. I'm 50-50 on this one. If you're a newbie in Markdown, you'd find yourself looking back and forth between the Markdown and the preview. But if you're a seasoned StackExchange user (and GitHub as well), then you'd know how it looks like by the syntax itself.
So we can have the third pane displaying the result OR...
We could have something like James McMinn's Writer app, which formats the raw Markdown itself to the style of the resulting text. It could remove the need for the third pane, or maybe set it as a toggle.
Now, I was thinking that it only showed the original post. But I come across a lot of answers that fork-off or add-to existing answers. So what if it also viewed existing answers as well?
I originally thought of this as a bookmarklet that just took out the parts of the page, and displayed it in a full-page modal on top of the page (optionally with the browser going fullscreen). That way, it doesn't break the StackExchange layout. Once done, it just sends back all the values to the page. Submits are done on the page rather than the modal.
Now this could go out to the other StackExchange sites as well, like StackOverflow or Programmers or even CodeGolf.