Markdown vs. HTML in a CMS
I vote for Markdown.
I picked up Markdown in maybe 5 minutes in writing my first response here. Later I learned more than what I picked up here, but I'd think this to be rather standard.
Markdown is much simpler to get good markup out of, and if you're worried about speed just cache the resulting output.
Markdown is often better, and more easily understood, in plain text than HTML is in a WYSIWYG editor. Also, no-script friendly.
And if you've got a user who wants an embeded object, just drop the HTML code from that Youtube video in and it'll get carried over.
If usability is an issue, and the target audience is non-geeks, WYSIWYG wins over Markdown. People are used to the toolbars with formatting buttons, but Markdown is a completely unknown markup language to most people (even "markup language" is completely unknown!).
I've had to explain a Markdown-lookalike wiki syntax to non-geeks at work, and they don't love it. When you want to write something, you want to write something, not look up weird ASCII syntax. Try not to interrupt the users' flow.
I would find a good WYSIWYG editor, like the one in WordPress (TinyMCE). It works ok.
If you wanted to use Markdown and a WYSIWYG editor you can use something like WMD Editor which (I am 99% sure) is what is used here at StackOverflow.
The benefit of using something like this is that your non-tech users get their WYSIWYG editor, your techie users get their Markdown love and you get clean markup. Another added side effect is it may actually teach end users Markdown from using it (or at least in an ideal world...)
WMD Editor also has an instant preview (which you can see when writing posts on StackOverflow), which will show users how changing the Markdown changes the look of their text.