So much this.

Normal non-technical people understand that you click a bold 'B' and you get bold text, you click an italic 'I' and you get italic text, you click a little list icon and you get a list, and so on.

contenteditable has its quirks, sure, but modern browsers mean it's no longer the brainmelt it once was. Real WYSIWYG entry is achievable and almost easy. Markdown has one great usecase (producing documents that are viewable both as plain ASCII and as 'compiled' rich text), but it's not a tool for Joe Normal.