What are experience points? What is the difference between second level, which takes 300-4000 xp (depending on class and edition) and third level, which requires 600-8000 more xp? To use 5e’s baseline Fighter, at second level we get Action Surge, which nets an extra action once/short rest, for 300 xp. At third, we get a martial archetype, which is Improved Critical, Combat Superiority, or some spell casting with Weapon Bond; which we get for 600 xp. Add in multiclassing, and if we go Fighter 2/Rogue 1, that 600 experience nets us a skill, proficiency with thieves tools, Expertise, 1d6, and Thieves Cant. It all seems kind of arbitrary, right?
At this point in time I think it’s important to remember a thing that we already know: experience points are an abstraction. A freshly created character starts with zero experience points, no matter what. It doesn’t matter if they’re a 90 year old Wizard or a 16 year old Cleric; whatever it is that experience points are measuring, they both start out with the same amount of it. If we take a look at older editions of D&D, there are some experience points awarded for the slaying of monsters, and dealing with traps; however they are awarded primarily for treasure acquired. From this we can infer that experience points are not a measure of combat prowess, or wit, or strong stats. They are a measure of getting treasure out from underground. It might take a certain amount of combat and trap foiling and villain beguiling to get to that treasure, but what matters is having what it takes to get it out, and experience points represent putting all this together to be better at adventuring.
A second level Fighter doesn’t gain Action Surge instantly; it’s the result of 300 points of experience tying together various tricks and techniques to get 12 seconds out of 6. The extra 1d6 damage a third level Rogue gets is the product of learning more about where vital points might be, how to twist the knife in such a way as to do the extra damage. That Rogue might have learned those techniques from a time where he was popping gems out of a statue while dangling from a frayed rope that snapped, causing the blade to lever in just the right way
This also helps explain the strangeness in multiclassing. Why does it take the same amount of experience points to go from Fighter 4 to Fighter 3/Rogue 1? Total level represents your ability to put it all together. A Fighter 3/Rogue 1 can put together more things in the stress of a dungeon than just a Rogue 1 can, and these things represent that gap.
So, next time someone complains about experience point systems, and how they can pick a lock or throw a punch, ask them how well they could do those things in the dungeon.