I read Monte's latest article on D&D 5e Its about Vancian Magic and how 5e is going to mix Vancian Magic with at will powers and what not.
From what I gather, it's not just adding a 4e type class to the caster's mix, but adding at will powers to the basic Wizard class - mixing at will and Vancian magic.
My "Old School" issue with at will powers is that the character with at will powers is never disarmed. Sure, a fighter without weapons can use his fists or improvise a weapon, but a caster that can always cast is never without an effective attack.
It goes against the whole "Vancian Magic" idea in the first place. Go with one or the other, but not both in the same caster. This isn't putting your peanut butter in my chocolate. Somethings just shouldn't be mixed.