The following a a few easy and simple ways to improve the easy-of-use of the mobile version of stackoverflow.com:
Increase the size of the navigation menu buttons These buttons are currently too small for a mobile user to use easily. By increasing the size of these buttons and making two rows, you could make it much easier to navigate the questions of the site.
Increase the font size of the
add commentlinks Making these links larger would make them easier to click on and read on small screens.
Either widen the Your Answer
textareabox to take up the whole width of the user's screen OR make the question's margins the same as the
textarea's. I'm not sure how other mobile browsers treat this input, but on Chrome and Safari on iPhone, a tap on the box zooms in on it. Since the box is currently not full-width, looking at the question after the zoom is annoying because it requires a pinch (to zoom out), and then a scroll.
I hope to see a few more suggestions for the mobile site and some comments on mine.