The removal of Reviews mainly affects developers that solicit reviews for their apps, such as game developers fishing for a five-star rating. By killing the Reviews feature, Facebook is signaling a commitment to surfacing “good” apps only through social discovery.
As for the Discussion tab, Facebook is now encouraging developers to make use of Posts and Comments on their Walls as the best way to encourage conversation:
Facebook’s latest platform updates also reveal a new API for retrieving Page Questions and signal the end of App Reviews and Discussions. The API update follows on a previous Questions modification where developers could retrieve questions asked by users via the Graph API. Now developers can access Questions using a Page.
Facebook also set out a list of breaking changes, including info on the OAuth spec migration, Deprecating Dashboard APIs, removing the Bookmark URL, and updating the FB.Canvas.getPageInfo method so that it must be called with a callback function.