Thumbs up on the Thumbs Up/Thumbs Down feature!
The call-to-action needs to be highlighted more, at least during the initial roll-out (when people aren't aware of the feature).
The current faded white-on-gray icons don't grab your attention and are in an awkward position. The adjustment could be as simple as moving the icons down by the other actions (Tags, Downloads, Share).
Love the tooltips (awesome vs. meh)