I think people should get some reward for weakening enemies, especially in teamwork scenarios.
Maybe I have a simple idea?
All servers know exactly how much damage A did to B in a given moment. It's a simple number!
So, suppose B lost X hitpoints.
We take this X damage, apply some factor to it (let's say, divide by 10 or 20) and then we award this to the A player.
This can apply to horse damage too, if the horse is "manned".
Simple?
To add some more individualism flavour, one could give some "bonus award" of Y points to whoever actually kills somebody.
As for gold... well, can be worked out, on the same principle.