Just to add to your point a, I'm often less interested in what a friend thinks about a book, and more interested in what another author thinks. Depending on who the review is by, it helps me understand the character of the book. If I'm unfamiliar with the reviewer, I might look into who the reviewer is and read their books. Dumbing this down into "my college roommate clicked like on this, so I'll like it too" degrades the entire experience for me.
Then I suppose you will be happy about this youtube change if it helps Google show you an author's comments? Perhaps because you +1'd them on google plus? Or because you bought one of their books?
This change, like it or don't like it, should not be conflated with, "Now Google will only show my comments from my friends." For better or worse, it means Google will know more about connections between people and the things they like. There are plusses (beyond seeing my friends' comments) and minuses to that.
