As another poster: The "signup for dropbox" is horribly intrusive. There's got to be a better way... say, a smaller window in the lower left corner that can be closed.
And, additional ideas from me:
A) If comments are already defaulted to off, then the sidebar should simply already be closed. I'd say that's by far the most logical and easy UX solution.
B ) The sidebar itself might actually just be plain unneeded. Unless there are already comments, it's a huge waste of space. It's likely that it should be a fixed toolbar at the top of the page. I mean, it's a whopping 5 buttons when closed. Why not just include those in the top toolbar that already exists, then if the comments button is enabled, allow the viewer to open up comments.