Personally, I think the whole challenge and enjoyment of playing a spell caster in older editions of D&D is the management of limited spell resources. I also think it's fairly obvious Monte Haul - er, I mean Monte Cook is paying lip service to the whole Vancian Magic schtick.
So, here's my thought - what if At Will Powers were "throttled" down? What if they weren't able to be consistently used round after round? What if At Will was actually Limited?
Say your at will power is Horrible Gamer's Breath (stay with me). Your first use in an encounter is automatically successful. The next attempt requires an ability check (INT if a wizzie). Pass the check, it goes off and you can try again next round at -1 penalty to your check. Fail, and you can't try again until the next encounter. Penalties reset at the end of the encounter.
At Will with limits. I could actually live with this I think ;)