I’ve recently been mulling over this very subject, because my company uses a CMS with a horrible GUI that completely mangles input, produces invalid markup, etc.
Markdown certainly looks the ‘easiest’ to learn, although I’m suspecting there’s a lot more to it than presented here (off to research later …)
In my experience, though, however simple the HTML subset, and however much training you give re: elements, attributes, valid nesting, etc., people will always struggle with that most fundamental of beasts: the humble hyperlink. Let’s be fair, to a non-developer, a URL is a pretty complex string of syntax. And editors simply resort to copy+pasting. If I were adressing a ‘low-tech’ audience, I’d seriously consider one of:
a) Denying any out-bound links + clever wiki-style auto-linking
b) Auto-linking allowing out-bound links via a search engine (or similar)
c) Robsust URL parsing looking for obvious errors