I’ve been reading C.S. Friedman’s Feast of Souls. It is pretty good, so far. In the world that Friedman creates, magic is powered by the spark of life. Most witches can only use their own life force to power their magic, which results in them tending to live shorter-than-normal lives.
This isn’t a new idea, at all. Novels are full of wizards who burn themselves out (though it is often temporary) or cause themselves physical harm by overextending themselves… reaching into themselves for reserves of power at the expense of injuring themselves.
Games don’t model this particularly well. Some try. Shadowrun (and some other games – maybe GURPS?) tend to force magic-using-types to roll or take stun damage after using magic. Theoretically, this could turn into lethal damage at some point (depending on the system).
Really, though, I find most of these systems unsatisfying. There tends to be too high a chance of taking damage from trivial uses of magic. The danger doesn’t match up with dramatic tension.
What I haven’t seen is a system that is really incremental and seductive – one that allows you to use minor magic with no real risk… but the more of it you use the greater the risk is for more powerful magics. I’d also like to see a system in which players could make a choice to undertake greater physical risks to pull off impressive magical feats (reaching into their reserves of strength).