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The core of this design is the 'Editing Page' (collaborative and non-collaborative), and it is shooting for a minimal design. It lays out the original webpage as the starting point for annotation (or editing) and editing is done directly on the page. It divides the whole page into sections, and allows multiple users to collaboratively edit a page as long as no two of them are editing the same section at the same time.
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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 tools are provided; the user has to click or select text in order for the editing buttons to show. A tooltip on mouse over saying 'Click to show edit options' may help, but may also be annoying if it is constantly showing and covering up the content of the webpage below it. A panel at the top or either side containing all possible editing buttons is another option that may also improve learnability of the editing feature.
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The design does not provide user any help information or functionality. A quick help page for new users may be useful.
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, which results in one more click per highlighting action. Making the most recent selected tool last until explicitly cancelled (by right click, for example) may help, but going back to learnability, that knowledge of 'right click to cancel current active tool' needs to be learned somehow.