All of the following is IMHO and for my own campaigns :

Mages summon extraplanar forces to create and destroy.
Priests use faith to influence the world around them.

Mages cannot heal because the body rejects anything supernatural. Power drawn
from planes of energy just aren't a body's idea of a good thing.
Mages can influence the world around a character in a sort of pseudo-healing
(time warping, necromancy, or even summon a bandage).

Priest's spell powers, while drawn from dieties, generally improve a
character's "karma" and inner strength. Healing for priests comes easily.
This is why most priests can heal regardless of the aspect of their diety.
Priests don't throw around pure power like mages can, though they do have
some limited control over elements.

Yeah I know there are plenty of spells that break/bend my guidelines, but as
long as the game feels like its following these rules, I'm happy. Of course,
this is why most priests are prefered over mages, priests work through others
while mages deal with forces most have never seen.

Anyway, feel free to fire away, lets see how well it holds up :).

- -joshua