[mntmoss]

The distinction is in quantity and types of assets. In 2D you usually have some bitmap and vector assets, collision data, animations, and scene structures to tie it all together. In 3D you have all of those things plus the texture/model/material distinctions, lights, skybox, and all the other details that come in as you add more rendering capabilities.

However, you can absolutely make 2D games that need lots of kinds of assets. RPGs happen to be one of them, in fact: The incidental details of adding inventory, NPCs, abilities, dialogue, etc. does add up. Every little message and description, every item's properties. You can ship a game by hardcoding much of it, but that's not going to scale with any substantial team. You need real data management, build processes, etc. Where assets interact with each other you get incidental complexity of the Dwarf Fortress bug kind, so where you can, you add static checks. The engine is built in tandem with the asset pipeline, in effect.

So - whether it's 2D or 3D, what actually matters is the assets. That's why even in the early 80's you had vector games with 3D effects in them; they just took approaches that simplified the assets and the resulting scenes.