In our organization, people review flyers and before we started using digital Proofing, some reviewers would use a stamp on physical printed paper to indicate they've reviewed each product on that flyer page.
Those users really don't like having to emulate that with the available tools, as it means there has be a comment attached to markup and it get's quite messy and time consuming for them. Look at this example:
They would like a markup tool that is just a stamp (maybe with some color and text/image/size options) that they can quickly click around the document to indicate they've reviewed/approved that individual part of the document (no comment required). It would also be helpful to have a toggle to turn on/off viewing of stamps.