Imnsho
The way I see them, the 4 roles are abstract statements about what a class focuses on. It doesn't limit a class, and a particular class will probably be able to do something in each category (they'll just be better at their role than any other). Also, I suspect (rather strongly!) that any character can be built in a way that moves it out of it's native role, too.
Also, the 4 roles are actually a grid:
........... Offensive.. Defensive
A Monster.. Strikers... Defenders
Battlespace Controllers Leaders
The idea being that each role is either primarily about changing one monster or changing the battlespace (terrain, creature formation, etc.). Beyond that, you can do so offensively (by hurting a creature or rearranging the terrain) or defensively (by blocking the same, or improving your team).
So:
Striker: damages/impairs specific monster targets
Defender: prevents damage/impairment to party
Controller: alters how the terrain/monsters are arranged
Leader: alters how the party is arranged
There's a third axis as well: how much else the character does. As such, a Fighter will likely do the most melee damage of any defender (because he does very little else). A Warlock will probably rule Strikers, too: Rogues/Rangers have a lot of extra functions they don't (based on what we've seen so far, anyway).
Over the lifetime of 4e, I suspect that more classes will be Strikers, followed closely by Controllers (there are lots of distinct ways to be both); Leaders will be 3rd, with Defenders probably being the least common type of class overall. Naturally, this speculation won't even be provable for years, so I have a long time to deny having said this
Anyway, that's how I view it.