For one, the class roles in 4th edition are all combat based. A character is more than what they can do in combat. I feel class roles are better defined at the Adventure level.
- Magic-User: The traditional adventuring role of the mage classes is to enable the party to get past tough encounters and progress further and deeper into the dungeon. They did this through their spells. Sleep/Web to subdue an encounter with superior numbers. Illusion based spells to distract a powerful foe to drop their guard or leave an area. Spells to teleport and tunnel through the walls themselves to get avoid encounters. Charming or reading the mind of the right enemy could get you access through difficult areas. Spells to weaken tough opponents, bringing them within the party's ability. And when all else failed, a properly placed and timed fireball could bring an encounter under control.
- Thief: The thief brought efficiency to the party's exploration. With their ability to scout and identify difficult areas as well as lucrative opportunities, the party could make good decisions on how to progress. Warning of danger with their ability to find traps, listen, and scout around hidden. They could also use their abilities to access areas and shortcuts normally out of reach through their skill to climb, unlock doors, bypass traps, and even pick pocket a key or key item.
- Cleric: The Cleric was the party's safety net. Their spells focused on healing, protection, and status removals allowed a party to recover from a misstep. When not specifically needed for their spells, a Cleric was still a capable front line fighter. Having a Cleric allowed for an extra margin of error for bad luck or bad play.
- Fighter: Where the other classes excel in specialized niches, the fighter was the class that held them all together. The Fighter protects the other members of the party by placing themselves in the heart of the battle, at its most dangerous point so the others can survive to when their own specialized skills were necessary. When faced with the unknown, a Fighter was the best choice to take point, as their incredible endurance would give them a better chance to survive any danger that was found.
The above is more hindsight of idealized roles. In reality, I do not recall building party's based on class roles, but they tended to be balanced because of everyone's need to feel unique and stand out. I did notice that certain classes attracted certain people or moods. A class was less a role, and more a state of mind, a different flavor to playing the game. A player should choose a class because of it's play style and skill, not because the group "needs" a particular class it is lacking. To this end, I offer up a revised set of "roles" for the 4 basic classes:
- Fighter: One should play a fighter if they want to be the heroic focus of the story. A fighter is about action, and always being in the center of things. Fighter's empower the player with strength they may not possess in reality, letting them take a confident lead in dangerous situations. Fighters are a straightforward class, good for players who do not want to worry to much about rule details and just want to roll dice and cheer.
- Thief: One should play a thief if they like living on the edge, relying on luck. Every situation is a chance of adventure to a Thief, where quick wit and tongue are stronger than steel. Trouble and drama finds Thieves at every turn. Thieves also walk in the gray between good and evil, affording the player more room to explore less savory means of interaction and problem solving. While a Thief often finds themselves the center of attention, they tend to be on the edge of the overall story.
- Magic-User: The Magic-User is the class for the thinker and problem solver. A well timed and purposed spell can completely alter the current situation. Your resources are limited though, so choosing the proper time of a spell is as important as choosing the spell in the first place. Mage's tend towards being mysterious and full of secret knowledge others would pale to hear. Playing a Magic-User lets a player sow mystery around them.
- Cleric: The tradition of the Cleric is played by the passive or selfless. A Cleric however grants a player a strong presence and respect from those encountered and the party itself. A Cleric is confident and assured they are following the will of their deity, and that this god will give them the ability to prevail. This empowers the player to play confidently and direct the group onward with great speeches when their will falters. Players of Clerics are confronted with moral issues more than any others.
I prefer basing class roles on play styles rather than justifying the existence of the class because it brings something unique the party needs to survive. Any party composition should be playable, especially when you have a small group (less than the number of roles). A player should play a class they enjoy and that inspires them, rather than feel constrained by what is practical or necessary.