The check box has a strong visual affordance for clicking; it appears as a pliant area because of a mouseover highlight or a 3D “recessed” visual treatment.
Almost any functionality can be moved there, and it can be set to automatically hide, and then reappear upon mouseover of the area.
That said, I appreciate the fact that avatars are not showing; you have to mouseover the tweet to see the avatar magically "appear" in the bottom right corner.
In Firefox’s fullscreen mode, toolbars and the tab strip are hiddenat the top of the screen and only shown on mouseover.
