I've only done a few questions here but already it's grinding on me. Why can't we have the writing-answer panel and the preview panel side by side, rather than below, this means for big answers I can't make use of the preview! It'd be great if side by side, two scroll-bars, or even a pop out (I have a window manager that gives me an always-on-top button :)) but this may not help those 'less fortunate' when it comes to OS.
It'd also be great if the preview wasn't trying to update EVERY keystroke, I'd love to have (the option, as I can see others disagreeing) an update button, or only when I pressed enter. Maybe only try and update everything after the last parsed expression (rather than the entire messages)
It can get QUITE laggy. So please please do this, or have like a profile option to do it.