I was really disappointed when I got my first couple of open world kills and wasn't rewarded with anything. Absence of a reward was a splash of cold water on the sense of warm fuzziness.
I don't understand why this would be so hard to balance properly... Just scale back the rewards so that you get more for killing a harder opponent, and nothing at all for killing an opponent that is under-leveled compared to yourself. Also scale back rewards so you don't get anything if you outnumbered your opponents too much.
Reward in commendations, credits, titles, and some kind of in-game statistical record of the enemy players you have killed and how many times per player. Throw in the other side - of which enemy players have also killed you, and how many times. Get some good vendetta's going. ^_^
Doesn't seem like it's that hard.
But then again I'm just a random twit on the internet. What do I know about MMO design?
Nothing, that's what.