Jono at Mozilla Labs:
The new icons are inferior to the old text buttons. The text buttons were self-describing. The new icons are not. I’m not usually a fan of toolbar icons; they’re never as self-explanatory as their designers think they are, so they usually need text labels to be decipherable. At that point, why not cut out the middleman and just show the text label instead of the icon?
I switched back to the text-labelled buttons as soon as it was possible. I, personally, wouldn’t mind the icon-labelled buttons if the icons weren’t so awful—in Gmail’s case, it’s like they scratched their head and struggled to think of something that would fit the button’s action, and chose something that was clumsily reminiscent of what the button actually does.