Posting in StackOverFlow isn't visual. Users have to fetch what it will look like and have to post. So, please add a feature of visual editing (Like Wordpress).
For me the current editor and preview are enough.
I do understand you won't be a markdown master from the start, but you don't need to. The toolbar helps you to learn the markdown language, and even if you don't learn the syntax, still the preview will show you what the end result was.
You seem to be asking for a WYSIWYG editor. There is no added value for such editor. Often I find those working not too well.