Classic examples of mistakes in the past are like "How to propose?" or "What's a good engagement ring size?" and so on. All these problems were exposed with Facebook Beacon and purchasing decisions and after much revolt they shut it.
I honestly can't understand why Quora would implemented "User X viewed User Y" - I think that's taking the privacy perspective to a whole new level. Indeed, even on Facebook if they started listing things like "User A viewed your profile 55 times today" - it would essentially kill the service in it's tracks as would "User B viewed this photo 33 times" and so on. People have always used Facebook to stalk their friends - but that doesn't mean it should be detailed publicly for all the world to see.
This should be "opt-in" if not removed all together in my mind. As part of internal metric tracking - it's obvious that this occurs - but it shouldn't be public or should be entirely opt-in.