I'd be wary of special-purpose editors. Generally devs are better served getting really, really good with one editor, and then extending it to meet their needs. Sublime Text, for example, has good Markdown syntax highlighting, and I'm sure it wouldn't be that hard to build live preview, assuming that such a plugin doesn't already exist.
That doesn't mean there isn't a place for a TTW editor - sometimes you just want to edit something in place when you're in a pinch. Nothing wrong with that. But I would argue in that case that you'd probably want a true WYSIWYG editor, a la what browsers kinda sorta give you with contenteditable containers.
I'm writing from a writer's perspective, not a developer's perspective. Which is not to say that there isn't value to what you're saying, but what I'm looking for is a good way to write quickly, not a code editor that also handles Markdown.
I want to convince the writers I edit on a daily basis to use Markdown because of its clear benefits over WYSIWYG. Sublime Text I'm sure is great and I've heard great things about it, but I'm going to have a far harder time selling the journalists I work with on that than I am with something like Mou or iA Writer.
WordPress' big weakness, IMHO, is that it has an editor which is not built for writing longer pieces and is poorly suited for editing. (Tumblr's big weakness is that it supports Markdown, but without frills.) That in my opinion is a huge issue—the words are basically the most important part of the editing process, and it means that in editing environments we're still stuck using Word or third-party tools like Editorially. What I see with Ghost is a real honest-to-God effort to focus on the editing tools first. We need more of that. It's where we spend most of our time on our blogging platforms. Why downplay that and instead put all of our energy into these inefficient WYSIWYG ContentEditable editors that nobody really, truly likes?
You'd be really, really wrong about that. Us writers hate using our mice when not necessary just like everyone else.
WYSIWYG can break up thought process when done poorly. And in the age of the iPad, nobody's really done it very well at all. It's kind of a relic because it doesn't translate well to touch.
Also, don't let the tagline fool you: Mou has a big audience among writers.
That doesn't mean there isn't a place for a TTW editor - sometimes you just want to edit something in place when you're in a pinch. Nothing wrong with that. But I would argue in that case that you'd probably want a true WYSIWYG editor, a la what browsers kinda sorta give you with contenteditable containers.