Well, when you hit reply you go to this editor with tons of buttons for formatting text. This is not something you're offered with the quick reply feature but that kinda makes sense because its the quick reply feature and isn't meant for more detailed posts. However, some of the buttons offered in the editor are still kinda helpful, mainly Bold, Italics, Underline, Hyperlink, Image, and "Toggle View". So maybe place those along the top of the Quick Post text box:
Additionally/Alternatively, if someone is writing a "Quick Post
TM" and then realizes that they need the full editor, have a button somewhere that goes to the editor with the content of the quick post box already in it. In the above picture I've placed such a button next to the Post button.
Additionally all of the above features could be applied to the interface when modifying a post. This is particularly important though because right now there is no way to assist posters trying to modify formatting issues.
The other idea is a little more low tech. It simply involves adding text to any editing area that looks something like this:
[b]bold[/b]
Italic[i]Italic[/i]
ect.ect.