I personally believe that one nice addition, whichever markup language is provided, would be a set of word processor-like buttons and err... well... keybindings to automate the insertion of some constructs: letting aside for a moment the fact that PM as for itself is much resistant to JavaScript, which in turn would be required for this feature, I'm also sure that the fact I'm now supporting it at least in principle, will literally astonish many people both in view of my past (and eternal!) advocacy for LaTeX and for taking exactly the opposite position in a similar discussion not too much time ago. (Unfortunately, I can't find the relevant link now.) The point is, both are not incompatible with the former: by actively blogging on my tumblelog and using a similar tool I verified that you won't loose your manhood by doing so!

FWIW (not much here, and I'm going slightly OT...) there are some considerations that I can do on those insertion methods; in particular Tumblr offers three input options: visual editor, raw HTML and markdown. With the visual editor you can still edit the HTML source of the text, by means of a suitable button. However the HTML is often "massaged," which is a major hassle and bothers me much, (but perhaps it has to do with intrinsic limitations of JS+HTML) and for some particular posts in which you want precise control over what you want to do, you have to switch to raw html. Sadly enough, raw html massages the code a little from what I have seen, too: probably due to "user friendliness." (E.g. straight double quotes get converted to opening and closing ones depending on the position relative to "words.")

Wikipedia has a set of buttons too, and they do not suffer the problems hinted to above: they just insert the appropriate markup. No more, no less. One very nice feature that I have seen in only one commenting system of a blog is realtime rendering, with restricted html as an input method.