First of all, this is awesome. Great idea, can't believe it hasn't been around till now, and will definitely be using it.
The only thing I could change here is the "x" icon when expanded. Someone mentioned here that they would be unnerved hitting an "x" button with a lot of text - this is definitely valid, I felt the same thing when trying it out. A simple change to an "exit full screen" icon (like this http://findicons.com/files/icons/1667/iconic/32/fullscreen_e...), would solve the problem nicely : )
I will second it. I would also move it on the bottom right, as I didn't look at it except with my peripheral vision and thought it was an X/close button. When instructions said to click it, I went from "why would I click the close button" to "oh, it's actually a fullscreen button" before I clicked it. I never would have discovered it.
The only thing I could change here is the "x" icon when expanded. Someone mentioned here that they would be unnerved hitting an "x" button with a lot of text - this is definitely valid, I felt the same thing when trying it out. A simple change to an "exit full screen" icon (like this http://findicons.com/files/icons/1667/iconic/32/fullscreen_e...), would solve the problem nicely : )