One of the things which early design articles talked about was the desire to give each class features which allowed them to fulfil their roles no matter what powers they took.
They did this for Cleric, Fighter, Paladin, Ranger, Rogue, Warlock and Warlord.
They didn't do this for Wizard, which is a huge shame. It would probably dramatically reduce the level of 'problems' which many people see with the wizard.
Class features are what (should, and in every other case do) define your ability to fill your role. Powers can be taken to support the role or to lean out towards some other role - most classes have a secondary role in mind.
Cheers