Roles are a little misunderstood. Like English grammar, they tend to be more descriptive than prescriptive- they don’t strictly limit a PC’s behavior in combat by themselves. Rather, they sum up the total effects of a PC’s class features and powers, which are focused on- but never strictly limited to- certain patterns. This is where 4e gives you room to have classes that serve the same general role but play in different ways, as well as encourage different builds within a class. A Defender is never just a Defender, a Striker is never just a Striker, etc.
But if we’re looking to build classes and powers, what are the roles giving us in game mechanical terms?