It’s strange running 4e and 5e campaigns at the same time. On the one hand, you can appreciate how elegant and streamlined 5e can be. On the other hand, you realise there is a lot of stuff – and I mean, a lot – that 4e does well. Stuff that adds to the game without adding to the complexity. Stuff that gets me thinking: could this be brought across?
One idea that I liked was each class having a power source. Fighters were different from Paladins because the former were martial and the latter, divine. It was a clear framework on how the characters tap into their powers. Martial characters train their bodies to superhuman levels. Divine characters invoke the powers of the gods. Psionic characters harness the power of the mind. And so forth. Compare that to 5e. How do characters tap into their powers? “Magic”.
I like having a framework. Magic is such a nebulous concept that rules and restrictions help. It explains why “magic” isn’t used to solve every problem or explain every phenomenon. And the more tight the framework, the more impressive it is when someone becomes powerful within it. Dumbledore is less impressive when you realise he can make up the rules as he goes.
Now, this power source concept is not perfect. The biggest issue with 4e’s take on it was that the power sources were meaningless labels. There was no real distinction between the primal and the martial, apart from flavour. All classes were built the same. The psionic power source changed the way those classes worked, and I suppose divine characters could Channel Divinity. But in general, the power source was almost irrelevant.