Actually, logged users can add comments on presentations inside Aha! but it may be interesting to allow non-Aha! users to comment a published presentation (public URL, restricted by password or not). Visually, it may be simply a Comments icon over the presentation webpage opening a popover view with the comments and an Add Comment option. I may be as simple as providing two inputs: name and comment.
This would allow clients to provide feedback on the slides and also managers not really accustomed to Aha! to do the same.
Documents that are created for internal review may go through a stage of external review from something like a Product Advisory board. It would be good to publish a document or "drawing board" item to them or anyone with that link, so they can add comments.
This could actually be made somewhat like a graphic design feedback tool: click anywhere on the report and post a comment that is contextual to those coordinates on the page.
If text, being able to highlight and provide in-line commenting would be great as well.