The checkbox button inserts a checkbox; if any text is selected, it is erased and replaced by a checkbox. Moreover, pressing Enter at the end of a line starting with a checkbox creates a new line which does not start with a checkbox. This makes it very slow to add checkboxes to a set of TODO items.
Instead, it should work as in Evernote, where the checkbox works like a list marker (from the point of view of the user). That is, the button should mark the current paragraph or the selected ones as list items with a checkbox as list marker. Moreover, a new paragraph after one starting with a checkbox should also start with a checkbox.
Additionally, the cursor should appear after a checkbox, not before, even though new characters appear after the checkbox.
Finally, I notice checkboxes do not line up correctly, which is (typographically speaking) rather ugly. This should be fixed.