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When the webpage is presented to the user, it tries to show affordances for editing by displaying section borders and the 'close' button on mouse over, but it is not obvious what editing capabilities are provided or what kind of annotation the user can do, because the user has to click or select text in order for the editing buttons to show. A panel at the top or the side containing all possible editing buttons may improve learnability.
In the collaborative editing mode, it may not be easy for the user to figure out exactly which part is not editable due to other's actions on the same page.
The design does not provide any help information that informs user where to get help.
Efficiency:
Making edit buttons just above cursor allows user to select an editing tool faster. It works especially well if the user is constantly switching editing tools. On the other hand, it may get tedious when the user is only using highlighting, and has to select the highlight tool every time after selecting text.