FFS, diminishing returns!
They are called EXPERIENCE points for a reason. They are MEANT to be a numerical representation of your character's life experiences. Highly intelligent life forms gain knowledge, and by extension power, through experiences.
Fighting is an experience. Fighting something for the first time and surviving is of much more value than fighting it for the 100th time. Naturally then, the points gained for fighting the same things over and over again, should have diminishing returns.
Another way to combat this is to set the levelling bars at continually higher and higher points, and then basically setting some kind of cut off for actually getting experience points. So a level 5 should get like 1 exp for killing a level 1 goblin, but get the right amount of points for killing something much stronger.
Of course gain EXP for quest related non combat stuff as well as skill checks etc., but removing it from killing things is retarded and betrays the actual concept of experience points entirely.
Look at it from a gameplay perspective. You have a quest to "retrieve" a certain item from a misbehaving merchant. There are several approaches you could take: you might intimidate him, you might beat him up, or you might steal the item. Each one of these approaches befit a certain kind of character, and reward you for your playstyle. You get the satisfaction of appropriately
roleplaying your character. The game
rewards your character build and the choices you made.
Now take a different quest - enter a drake lair to kill a powerful dragon matron. Suppose you have three options to complete the quest: you can pay or convince an experienced band of adventurers to clear the dungeon for you, you could sneak past the enemies and cleverly activate a trap that collapses the entire lair into rubble, or you could straightforwardly challenge the matron and her minions. Presumably here too we have the option of three distinct playstyles, and the game rewards your character build etc. But the truth is, that in this scenario there is one crucial difference - one option is substantially more rewarding than the others. Challenging the lair straight-on is the option that gives you the most exp and loot. So what do you think the player's gonna do? Well, you bet your fucking ass both the charismatic, manipulative merchant and the clever thief are gonna be donning platemail tonight. Characterisation gets thrown out the window, enter a shitty min-maxing metagame.