What I'm really looking for is almost a clone of GitHub's issue comment box.
Key features are:
- Works in all major browsers (very important)
- Upload files by drag / drop, clicking a link, or pasting (very important)
- Icon support for common formatting (nice but not necessary)
Also for the file uploader, it's really important that it works similar to GitHub where after supplying the file, it will inject the markdown image syntax with the absolute URL to the uploaded file directly into the editor.
jQuery dependency would also be fine if it comes down to it.