I prefer to keep magic a fairly mysterious thing, something that can be used by those who know how, but isn't really understood by anyone, beyond some vague and unproven theories that only scratch the surface. Not that some arrogant wizard couldn't come to believe that he's got it all figured out, though

I also like magic to be a bit unpredictable, just enough so as to keep it from being used casually without some consideration (except by truly reckless sorcerers), but not so much that it would be too dangerous to be relied on when you really need it.

I like the idea that excessive use of magic can fatigue the user or affect them in some other way, such as by burning them. Basically each magician would have a certain limit (depending on their skill or talent) on how much and how quickly they can draw upon magical power. As long as they stay under the limit they won't suffer any harm, but if they go over it they risk exhausting or hurting themselves.

Magic as "life force" could have interesting repercussions. If all living beings possess it but only sorcerers are able to manipulate it, unscrupulous ones may feel tempted to exploit other beings as a source of magical fuel.