Here are some things I would like to see with a tool like this:
1. The ability to choose what side of the circle the bubble of text is
2. an opacity slider so you don't have to toggle browsing mode on or move things around to see what's behind
3. drawing capabilities so you can mark an area etc.
4. same as 3 but shapes that you can resize
5. allow people to comment on feedback given - basically threads for each feedback bubble or whatever - that way you can ask for clarity etc.
6. an archive of all feedback that has existed - so you can track what feedback has been resolved etc.
7. labels that can be added to feedback bubbles and then some way to filter them all so you only see feedback that targets your specific label filter ex. you only want to see feedback for text or only feedback for design etc. can also be used for prioritization, as not all feedback should be treated equally
I am sure I could come up with more, but these are what I just thought about now.