Sorry for bumping an old thread, but this topic is always relevant.
My opinion is that "experience" comes from learning something. If a first level fighter kills a goblin, he learns something. If a 20th level fighter kills a goblin on the first blow without taking any defense, then they didn't learn anything therefore there is no experience gained.
If a 10th level wizard casts a fireball every single encounter since he's learned the spell, and he uses a fireball to kill a dozen goblins without ever being in any danger, then he doesn't gain any experience.
Now if that 10th level fighter takes three rounds to kill the goblin and the goblin hits the fighter three times, then the fighter is still learning something and gains some experience from the fight.
If the 10th level wizard has only ever cast the fireball on two or three goblins and is trying to figure out how many he can get, then he is gaining experience through the learning process.
If a rogue finds a trap and avoids it, he gets experience. If he finds the same type of trap again, maybe he still gets experience. If he finds the exact same type of trap at every door and can get around it every single time with no effort at all, there is no experience being acquired.
In my opinion, the XP of any encounter or situation is variable and flexible based on the unique situation.