4e is a complex soup, with lots of ingredients. MMOs (and WoW) is one of them, and it's one that a lot of people point to when talking about it, but it's by no means the dominant one.
And while roles primarily come from the classic fighter/cleric/rogue/wizard classes, there are two main differences in 4e. One is to make sure that if a class is supposed to do a thing, it
can do that thing. Fighters and paladins are defenders, so they have both the ability to take a lot of punishment and the ability to make enemies focus on them (or pay the price). Rangers are
not defenders, so while they can take a hit or two they can't be the main tank, and the game communicates that very clearly. Clerics and warlords are both leaders, so they buff and heal their allies. Druids are
not leaders, so they can't do in-combat healing (or at least not very well).
The other is that other editions have been less clear on these issues, and often positioned classes in between – e.g. paladins not being as good at fighting as a fighter, nor as good at healing as a cleric. Or druids having access to healing spells but on a delay, meaning they won't be as good at it as a cleric. Remember Ron Swanson's advice – never half-ass two things. 4e classes are built role-first – I imagine the design process went something like this:
- OK, so what would a primal striker be?
- A barbarian, huh? That's an interesting take.
- How can we make a class that uses primal power to deal lots of damage and does so in a barbarian style?
This would be unlike other editions where the primary question is "What should a barbarian be able to do" and whether or not they can fulfill a "role" or not is at best a secondary concern.