The DnD Wandering Monsters article this week is all about how many goblins you have to kill to gain a level. The very idea that this is the standard metric makes me giggle. Getting better at picking pockets, learning about the arcane mysteries of the world, or being better able to channel the energy of a diety you worship is all about how many monstrous humanoids you have watched your friends butcher. Obviously. There is a sound argument that brawling makes you better at brawling but shouldn't people then get loads of experience for hanging around the castle for months doing drills with the local weaponmaster?
I just don't see how you can engineer a system of gaining XP around body counts and have it work. This is probably my bias showing because in my games people don't spend every session doing fight after endless fight through a nonsensical dungeon. The world is not full of enormous buildings that contain ten neatly packaged encounters evenly spaced throughout and the inhabitants tend to do things like run away and get reinforcements when they are outgunned. In the last session I ran there were two fights - one which was an enemy trying to fly away from the castle window using magic while the players desperately tried to take her down, the other which was a brawl to the death on an austere mountaintop sanctum with a cult leader. Both of the fights were very important though neither was terribly threatening by the numbers. I just don't see the point of having characters spend hours murdering nameless mooks with no plot motivation to exist solely to justify level increases.
It comes down to the game being a combat simulator or a story. I like to tell stories and I like to encourage the PCs to find creative ways to solve problems. You can fight the cultists, convince them to go home, get the king to send the army after them, whatever. The goal is to solve the cultist problem, not to cut them open to get the XP stored in their bellies. Accomplishing goals in the story and doing things that matter in the world are what triggers levelling up in my game (that, and they generally just level up every two sessions...) and although there are fairly regular fights those fights are there because they make sense, not just as filler. The characters get better because they are working their own way through a story. In the simulator model of RPGs the GM just rolls dice and provides a world and the characters need to make their own advancement. That does require some fairly serious metagaming though as surely the characters, being intelligent and all, would figure out that they get better by murdering and would go find weak opponents to smash to get more powerful. See that? When the players wanted to go farm goblins for XP they weren't be metagamers, they were just having their characters respond to the way the world works in a sensible fashion.
There are a lot of things about the standard DnD model for levelling up that bother me. The presumption that early levels go by in a flash but later levels take forever is an unappealing artifact of the way the magic system scales, the fact that the system is based around a necessity of constant murder is a seriously problematic design constraint, and that all of this necessarily leads to immense dungeons filled with random and illogical inhabitants makes immersion much more challenging. People say that online RPGs ruined DnD but I think in many ways it is the reverse. Online RPGs grasped the essence of old school DnD and in doing so they created many of the absurdities that we all take for granted in that genre.