There can be 'just magic' with different classes using very different methods for harnessing and controlling that magic. Just because divine casters get their power form their deity, a Warlock from their patron, and an arcane caster from mechanistic control (etc etc) doesn't mean the magic itself is different. Even the effect could be different without changing that. Really, the metaphysics doesn't matter much beyond flavor text. In order for the types of magic to feel different it would really have to feel different to use them, which in terms of an rpg means mechanics. D&D gestures in that direction with the memorization and ritual rules, but it's a very weak differentiation.