I run a collaborative writing forum and, newly obsessed with markdown at the time, I figured that since users were mostly writing many paragraphs of text, they could pick up the markdown basics if a toolbar + cheatsheet helped them out. In my head, it was going to be some sort of amazing improvement and my forum community would begin chanting my name as they experienced the splendors of markdown.
Classic mistake.
Yeah, sure -- the writers could certainly grasp markdown #headers, how two linebreaks become a new `<p>`, and even `[]()` vs `()[]` links (with toolbar/cheatsheet help). Great! But nested elements and pretty much anything more complicated were a real monkey wrench or just plain unsupported.
Writers would put great effort into their posts (just like a blogger would), changing colors, right-aligning, center-aligning, changing font sizes. -- Things that markdown just doesn't support without some extending or post-processing that would turn into something far more confounding than the intuitive bbcode you started with.
Everyone just understands:
[center][size="6"][color="red"]My Cat, Mittens:[/color][/size][/center]
[img]http://imgur.com/mittens.jpg[/img]
[size="3"]Mittens says: [i]"Meow!"[/i][/size]