I'd like to think that Valve would implement such features, but I fear that they have such a good thing going with Steam right now that they don't want to risk publisher support by implementing sharing and trading of games. EA already abandoned them for Origin, Ubisoft have one foot out the door with Uplay, so if they loosen up on the DRM then the big publishers may walk. Or maybe such features would necessitate the end of the big Steam sales, which have been a huge boon for their platform.
Now that Microsoft appear to be introducing the concept of sharing digital games with friends, maybe Steam will be forced into doing it too. Or maybe the Xbox One will crash and burn and nobody will ever speak of it again.