In recent years I've been recommending WP as a CMS to many of my clients. They are happy with the overall functionality, but there is one issue that I've been getting a lot of complaints about. To a non-technical person WYSIWYG implies "what I see in a post box is what will appear on a page", but it is not so in WP. And some people find it very confusing when headlines that should be red appear black and in an entirely different font.
I peaked at the source code to see if I can "hack" it to suit my needs, and after some trial and error was able to apply my stylesheet to the TinyMCE container - that solved half of my problem. The other half is getting all tags in "Format" dropdown to conform to the same stylesheet.
So here is my feedback/request for upcoming versions: Why not change the dropdown to use actual tags? Instead of span class "mce_h1" use H1, and so on... That way when I style H1 it will automatically reflect in the dropdown, and there will be no more confusion :)