With inline LaTeX previews, we're already surprisingly close. In fact, I'd say that going all the way would be almost a step back. WYSIWYG is ultimately not an ideal editing paradigm: it wins in the short term, being easy to learn, but drags you down in the long term. I've recently started using Quora a bit more. Unlike StackOverflow, they use a WYSIWYG editor. I've found this significantly less convenient than Stack…
That --in the extend that it happens-- is a byproduct of the limitations of current WYSIWYG editors, not something inherent in the idea of WYSIWYG editing.
It's not like we had a lot of brainstorming and innovating solutions competing in this area (in fact, there are only 3, all too similar, major products: Word, Pages and Open Office, of which one has 90% of the users).
Second, what's "inconvenient" and "distracting" for one user, is a must and an inspiration for another.
Not to mention there are several different use cases for WYSIWYG editing, and LaTeX style editing doesn't cover them all. For example it's dreadful for quick experimentation with placement and formatting of small, disparate elements (basically, anything not according to the "spec").
>I've recently started using Quora a bit more. Unlike StackOverflow, they use a WYSIWYG editor. I've found this significantly less convenient than StackOverflow's markdown.
WYSIWYG's strong point is not small website comments.