I personally wanted to have this "xp rate" to regenerate over time, so that the lowered xp amount is reseted each day or such. This way it would have lowered the very high creature farming rate (killing centaurs and more centaurs and more centaurs and even more centaurs,....), without ending in permanent 1xp per creature. Dunno why this never was done or tried.
Well, as said, an "achievement" system could do the magic.
Chi si ritira dalla lottaE' un gran figlio di mignotta!
But the sense behind it exactly has been that you shouldn't farm all day a certain creature to level up. Players will always do this if the gain is high, which was the reason for the idea at all: To make players look for other ways to do gain xp and of course to slow down the leveling process.