The dictum to set them to be "optional, ungraded, or droppable" seems pretty scary to me - if I were an instructor running a course where the graded work needed to be peer assessed (e.g. essays), I'd wonder what exactly was expected to go wrong the first few times that means I shouldn't have mandatory graded assignments that use it.  E.g. is the technology expected to fail?  Or is there a pedagogical mistake I'm expected to make the first time I use it?  Given that most users of the feature probably need to make the assignments mandatory and graded, I wonder if this is more scary than needed?