Well, why don't we look at the actual game mechanics that are given to a specific class to have it become a member of that Role.
The Defender classes: All have a Marking ability, and all have a punishment ability that occurs when the Mark is disregarded. They also tend to have high AC and high hit points.
The Striker classes: All have abilities or power add-ons that add extra damage or extra dice of damage on top of the normal damage their powers do.
The Leader classes: All have a 'Word'-esque power that allows them to 'heal' twice in an encounter. They also have powers that tend to buff their compatriots or let them do extra helpful things.
The Controller classes: All tend to have more frequent use of AoE abilities and attacks, and more abilities that debuff the enemies on the battlefield close and at range.
Now let's be honest here... is there any reason these abilities for these "roles" couldn't get layered on top of more classes than what 4E has? No, not at all. There's no reason why a Druid with an animal companion couldn't have a defender's marking mechanics for the companion, or why a Wizard couldn't have additional striker damage on their evocation spells (at the expense of losing some of his debuffing spells), or why a Monk couldn't lay on hands, or a Rogue couldn't control the battlefield with poisons, smokebombs, caltrops, alchemical fire and the like. After all... we've already seen this in Essentials, with some classes getting new roles (the Hunter Ranger being a controller, the Slayer Fighter being a striker, and the Sentinel Druid being a leader.) These choices don't necessarily need to be hardwired automatically into any specific class... the abilities that lend themselves to a certain role could easily be made available to all classes and chosen by the player during character creation to help do what they want to do with their character. So that if the Cleric of the God of War wants to be heavily armored and punish those enemies who move away from him on the field of battle, can have the ability to do so.
The Defender classes: All have a Marking ability, and all have a punishment ability that occurs when the Mark is disregarded. They also tend to have high AC and high hit points.
The Striker classes: All have abilities or power add-ons that add extra damage or extra dice of damage on top of the normal damage their powers do.
The Leader classes: All have a 'Word'-esque power that allows them to 'heal' twice in an encounter. They also have powers that tend to buff their compatriots or let them do extra helpful things.
The Controller classes: All tend to have more frequent use of AoE abilities and attacks, and more abilities that debuff the enemies on the battlefield close and at range.
Now let's be honest here... is there any reason these abilities for these "roles" couldn't get layered on top of more classes than what 4E has? No, not at all. There's no reason why a Druid with an animal companion couldn't have a defender's marking mechanics for the companion, or why a Wizard couldn't have additional striker damage on their evocation spells (at the expense of losing some of his debuffing spells), or why a Monk couldn't lay on hands, or a Rogue couldn't control the battlefield with poisons, smokebombs, caltrops, alchemical fire and the like. After all... we've already seen this in Essentials, with some classes getting new roles (the Hunter Ranger being a controller, the Slayer Fighter being a striker, and the Sentinel Druid being a leader.) These choices don't necessarily need to be hardwired automatically into any specific class... the abilities that lend themselves to a certain role could easily be made available to all classes and chosen by the player during character creation to help do what they want to do with their character. So that if the Cleric of the God of War wants to be heavily armored and punish those enemies who move away from him on the field of battle, can have the ability to do so.