The achievements solves part of the user activity issue. but the other issue about rating is still open. Does anyone know of any reputation or Karma plugin for buddypress. I guess it matters mostly around the forum. I found a plugin for bbpress https://bbpress.org/plugins/topic/bb-reputation/ and also there is intense debate for blog comments.
I will be needing this, so any advice on how to go forward to write or adapt a plugin would be incredibly helpful.