For example, wide screenshots become unreadable in the preview pane.
I’ve realized that part of the issue is that the current preview pane is only a preview of the Editor pane, not actually a preview of how the forum post will appear in the current window. Normally, where I can have a preview, it is as the post will actually be displayed which is not what we get here because it is crammed into a smaller box.
I do realize that many of the forums and message boards I’ve used don’t even have anything worth previewing so the current situation is better than most. But Discourse isn’t one of those forums so expectations are higher.