Relaxation and fondness should be reflected by nonverbal and paraverbal cues (facial expression, posture, gesture, tone of voice, etc.). They should be joined by an "interest/boredom" scale, for sure.
Implementing a conversation system like this well would create a nice form of alternative action to combat-oriented gameplay, using many of the same twitch reflexes. Buttons could be used to cause your character to interrupt someone, to keep talking when interrupted, to make gestures and control other non/para-verbals, to stop yourself mid-sentence, etc.
It wouldn't be any more complex to script than an MMO combat engine - the only question is, would it be as lucrative? Developer persistence and interest level is directly proportional (overall, usually) to expected monetary gain. IMO, properly developed and marketed, this kind of game could be a huge draw to many people who don't currently play a lot of video games. Requiring you to moment-by-moment manage your emotional connections with other characters as opposed to your life versus your enemies', but with the same level of immersive and challenging game play, could be a significant draw to potential female players and potential older players - two demographic descriptors for whose groups the "casual" (mostly puzzle-oriented) games are currently the only offering.