I find myself constantly switching back and forth from the WYSIWYG editor and the Plain one when adding media like FLASH SWF, and FLV. If I don't the WYSIWYG will strip down the code and it wont show at all in the editor.
So a Flash Button to add a swf embedded file to a post would make things so much easier. All you would have to do is upload the swf file and the rest is done for you.
A spell Check would also be very helpful for the Typo Freaks out there like myself.
Editor Monkey has a good system, but unfortunately that is no longer in development.