More feedback to the player
I think you should be able to see what happens when you kill a creep like in dota you can see the money pop up. I can't tell why its good to kill creeps other then to push but when you bo berserk angainst creeps you get stronger in some way. atleast I get that feeling.
If there is no bonus for killing creeps there should be since otherwise it would be a waste of time compared to killing heroes only.