I'll go out on a limb and say I'm not a huge fan of markdown. I think the composer should really be like gmail etc with rich text. Leave the markdown to the coders IMO.
There, I said it drops mic
(Okay without being too controversial here, yes you can accomplish the above with a plugin and no, we're probably not changing the default anytime soon :p)
I agree to an extent. Markdown is very powerful if you know it, but it can be very different for people who have never used it.
I think just adding more options to the editor would be handy. Sort of like what some of the php forums offer.