I could tell what buttons did simply from the text (and they were useful) e.g 'unwatch', 'your fork' etc. Now there's more space taken up with the text and the icon.

The icons don't seem to provide additional info in any way. Perhaps that's because Github would like to switch from text to icons eventually (with text+icons in the interim)? I hope not, as it would mean way too many things for people (esp novices) to get their heads around.