In game bounties can be very fun. In Shadowbane when we played it for a short time, I played an assassin (Big surprise), but I actually strictly roleplayed it in game, i.e. I only killed people for a reason i.e. in city siege battles, I would spend hours infiltrating the opposition city, get close to the guild/city leader and take my shot at killing them to disrupt city defense. I used "enemy" forums to post assassination notices. Quick way to make you a target, but from that aspect of using game mechanics (as a one shot kill for an assassin was possible), was a great deal of fun for me.
Too bad you can't combine all the best elements of all the MMPOG's out there