4E: Cost in Stamina

I'm not a big fan of fire and forget powers, but those that wear down a resource over time I can get behind.   So, harking back to the reserves mechanic, a random idea:
Using an at will power is same as usual
Using an encounter power costs a number of hit points equal to the power level.
Using a daily power costs a number of hit points equal to twice the power level.

Again, this is is an attempt to not keep hit points from simply representing ablative damage endurance, and return them to being what they've nominally represented all along:  stamina and accumulated luck.  In this case they can also represent mystical sway.