Someone grab a starving humanities grad student and get him to write his dissertation on this article.
Seems like you could say that 4e took a more "Postmodernist" approach to game design. Basically, when writing up the system, they said, "We all know that we're just playing a game and trying to accomplish something through that, so let's call a spade a spade and acknowledge all of the game elements as the constructs that they are."
The way I see it, you can go the illusionism route and say that the game mechanics rise out of the reality of the fantasy world, but if you look beyond that then you'll see that those realities exist in the first place to serve a purpose in the game. The game world and its lore don't exist until somebody thinks it up and then puts it into the game, and the designers don't bother inserting something into the fiction unless it has a useful function.
So, throughout the whole history of the game, you've had creatures that were long-range attackers with effects that have an effect over an area and control the battlefield ("Controllers"): Wizards, liches, etc; creatures that are hard to kill and do a lot of damage ("Soldiers"): fighters, owlbears, orc warchiefs... etc. All 4e did was codify these basic trends to make it easier on DMs and help them better understand and make the most out of the information they're given on monsters' stat blocks, etc.
I have a question for 4E players & DMs.
Do 4E monsters strictly conform to roles? Are monsters with the same roles likely to have similar powers and vulnerabilities?
(For example, take the ogre mage. I can't find its 4E role on the internet--and I don't own the MM--but does this monster conform to a single role? And if, as a player, I know its role, do I know more or less what to expect from it in a fight?)
Well, first off, unless you have seen the monster's stat block, you don't necessarily know what its role is going to be when you first see it.
Roles are sort of a "big tent" thing. There are only a handful of them, so there is a lot of room for variety within each one. All a role tells you is a monster's basic strategy and vulnerabilities: Brutes do a lot of damage but are easy to hit, Controllers control the battlefield but are physically weak, Skirmishers are easy to hit but highly mobile, etc. Beyond that basic idea, you won't know how exactly the monster is going to achieve that until you see it in action. A controller might have a lot of AoE fireballs, or it might daze enemies and dominate the others to make them attack their dazed allies; a skirmisher might jump halfway across a large room and throw a spear, or it might shift between a group of its enemies and hit each one along the way.