We use Markdown for text formatting on all Stack Exchange sites. Markdown isn't difficult to figure out, particularly since it apes common ASCII formatting conventions -- and its simplicity means it is amenable to wiki style differencing and editing, which is a big part of our engine. But that doesn't mean we can't do a better job of helping new users figure Markdown out.
To that end, we recently added inline comment help which explains the limited subset of Markdown supported in comments, and how to notify other commenters that you're talking to them. Click "show help" to expand inline comment help, or if you're a new user, it will be pre-expanded for you.
The brand new inline post help we just deployed is much more extensive -- posts support the full Markdown spec, such as it is, and even a whitelisted subset of HTML tags. Click the little [?] icon to expand inline help -- or if you're a new user, it will be pre-expanded for you (to the first tab level only).
We hope this improved inline help, compared to the rather clunky external help we had before, will lead directly to better formatted, easier to read, diff, and edit posts for everyone.
I know, probably won't happen, but like Parappa the Rapper, I gotta believe!