4th edition introduced a new way of looking at characters in pen & paper rpgs, that of the "role". These roles weren't anything overly new to the game, but the labels were. These labels were borrowed from MMO terminology, modified to fit into the construct of a table top game, and let loose upon the world of Dungeons and Dragon's players for good or ill.
Some people used the codifying of roles as another example that 4e was "just an MMO" in paper form, forgetting that MMO's spawned from table top RPGs themselves, first as MUDs (Multi User Dungeons) and related games, and then as graphical worlds in and of themselves, each one improving (hopefully) on the last. The conceits of D&D and other fantasy RPGs were borrowed and are evident in Everquest, Ultima, and yes even in World of Warcraft.
These roles are fairly simple, for the most part.
Defender: The "tank" in MMO speak, the defender punishes the enemies for attacking his friends.
Striker: The "DPS (Damage Per Second)" class in MMO speak, also known as the guy that puts out the hurt.
Leader: The "healer" role who helps his team mates by replenishing hit points and tossing out bonuses.
Controller: the "crowd control" role in MMOs, able to impose negative conditions on enemies, and generally make the enemies life miserable.
These roles aren't all bad, in one way or another they've always existed in D&D. The fighter, for the most part, has stood in front of the mage to prevent the monsters from interrupting his spells (acting as a defender). The rogue sneak attacks the bad guys, doing extra damage (being a striker). Clerics heal, wizards cast fireballs, and so on and so forth. What MMOs had to do was enforce what was already there and make those things mechanically work within the boundaries of a video game.
What 4e did was create the labels as both a way of discussing what a class was supposed to be doing and also a way to compare classes meaningfully. If strikers are supposed to deal damage, we can compare 2 striker classes, and look at how they are accomplishing this goal, and see if its working or not. No matter how you feel about roles, from a design perspective I can't see this as being anything but useful. At least, if you value class balance. Another idea borrowed from MMO and video games. To clarify, class balance means that at any given level, 2 classes doing the same job are relatively comparable and viable. You don't want X to be so much better than Y that Y is almost never played.
Roles can also be restrictive, though. There were many people who played the fighter class as a damage dealer, choosing to be "the guy in lots of armor with the big sword". Where 4e initially went wrong was not providing options for players who liked to play a class against its stated role. And, though I love 4e and playing it with my friends, that is true. If you want to play a pure damage dealing wizard...you play a sorcerer and deal with it. That's not a game breaker for me, but for others I can see the frustration. It has made steps in the right direction with essentials and with options like the Heroes of the Feywild barbarian, but for some that is not enough.
In the Next Edition should roles stay? Yes, I think they should. Not only are they a good way to decide what type of class to play, but also a way of achieving balance. It is a good way of offering comparisons, deciding on features, and also a way to find out what a player wants to do. Some people like playing the healer, other guys just want make things bleed. Keep them. But they shouldn't be a straight jacket for classes, they should be a descriptor, flexible, and those descriptions should be able to change depending on what options you take. If they stay with Powers / Abilities perhaps each one could come with a descriptor, and then you decide what your character is by the balance of powers you take?
IE: You take 3 powers with the "Striker" label, and only 1 "Controller" power, and your character is considered a striker. Or, choose this class feature and you are a Striker, and this one and you are a Controller, or Defender.
In current 4e terminology a fighter could look like this
If you want to play a defender choose Defender Aura for your class feature. If you want to play a striker, choose Power Strike as a class feature. If you want to play a controller, choose (This Thing) as your class feature.
Not perfect, I know. But it works for the purposes of discussion.
Now, the second broad topic I wanted to discuss was archetypes are something that are very useful in D&D, and in literature in general. In some ways Power Source is an archetype in 4e. Roles are also a form of archetype.
But more broadly than that, archetype is a general descriptor that lets you know what flavor a character has. If I ask you to describe a "Rogue" you have a good idea of what that type of character is in your head. If I ask you to describe a "Mage" you also have a good mental image of what that character may look like. These images persist through storytelling, MMOs, table top games, literature, artwork, and almost anything relating to our hobby.
I posit that there are several prevailing archetypes in our hobby:
The Warrior: A person who takes up weapons and armor to engage in hand to hand combat.
The Sneak: A person who relies on stealth and deception. Lightly armored and armed, the sneak keeps to the shadows.
The Spellcaster: Wearing no armor, the spell caster wields power unknown to the common man to smite her enemies.
The Healer: The healer also wields power, but unlike the Spellcaster who uses it to kill his foe the Healer uses his powers to aid his allies.
The Archer: Just as it sounds, the Archer prefers to fight at range.
The Savage: A person of the wild, the savage shuns civilization and its traps.
Now, many archetypes are missing from this list, and others can be seen as a combination of the 2. A ranger could be seen as a Savage Archer, an Archer Sneak, or a Savage Archer Sneak. Notice that even the term "ranger" brings up a set of preconceptions when I mention it? That is a type of archetype. A paladin has a clear archetype as well, a warrior of the divine. (an archetype mixed with a power source).
Archetypes, power sources, and roles are all building blocks used to describe and differentiate characters. Race can be seen as another block, each race having its own archetypes built in either mechanically or historically.
I hope that in the next edition that we have access to all of these tools in order to build interesting stories, discuss balance and mechanics, and work towards design goals.