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I would expect that most everyday Internet users, if they were asked to try out both Markdown and a WYSIWYG editor, would prefer the WYSIWYG hands down.
Even being a coder myself, I tried to get into Markdown to write articles for my WordPress site, but ended up returning to the visual TinyMCE editor. I just found it more convenient.
Markdown's popularity is due to 'geek appeal', not ease of use. The masses won't appreciate it the way coders do.