I like that reviewers and co-authors can see comments, but I'd also like the ability to have these not be displayed to readers.  For me, "Readers" are customers and end users, whereas co-authors are other employees of my company.  We might need to make comments and discuss a document in progress without that conversation visible to end users, although the end users are welcome to see the most recent version of the text itself.

This would be as easy as adding a check box to the sharing panel:  "Hide comments from Readers?"