Failiure and the wear and tear of magic was something else I also considered in my notes (really!), consider it, in races where magic isn't an everyday thing (a simple tag setting the chance of a species member being born with magic is all thats needed for some races to be 100% composed of mages).
Lets see...
Given that a spell needs focus in order to cast it properly, naturally it requires brain power, maybe even a significant amount of development in the brain is needed for those races where magic is simply rare and their minds rarely can be capable of handling such output in a way (I have been aware of the new stats, but forgot them and the location of the file). So over time as they use magic more their brains would develop new synapses and such, the begin to use magic with more efficiency for longer, though as this goes on the output of chemicals in the brain increases somewhat, such that they develop an addiction to the higher levels, but to do that they need to use the synapses they specifically developed for, they become addicted to magic.
Sure thats ok, not like they aren't supposed to use it aren't they? Though this means as the addiction becomes more severe they need to use magic more frequently and their morals about how their spells should be used become somewhat warped (as expected evil mages might not mind so much...). Eventually from a long life of using magic they go mad and beserk everywhere with their magic gaining happy thoughts galore until they die.
Naturally though the way to deal with this is to treat the addiction (MUA?), by not using magic so frequently they don't get so many cravings, but are not so useful as before. Though there is another underlying cause, reaching beyond their own capacity to use magic that demands greater mental resources than they can give corrupts the spell they cast reducing the likelyhood of it casting (something we already established I think), as a result though their brain increases the output of the synapses dedicated to magic and makes more of them, to increase 'brain power' to increase the magic user's competency.
Other consequences of their minds being warped is that they also loose coherency and rationality, meaning they panic and worry about their own mortality and so are inclined to accept offered deals to ensure immortality. (Hopefully the demons will only give them immortality after they have wiped your fort out =P)
I find the idea of demons eating your soul and maybe taking the body over interesting one as well though.
And stats related to the use of magic... recuperation sounds like a good subject for recovering from the use off magic and 'recharging the batteries', focus and endurance together sound like the primary points of how long magic is used for, while toughness and willpower sound like good candidates for 'overtime', I can see the other stats having varied use, not all used at once, but for different kinds of magic (on the casting and effect end of these sticks). Though I guess analytical ability, memory, creativity and intuition would be more about learning, remembering and making spells.
Though admittedly with all the core stats having their uses I guess talking about what ought to qualify a mage to use magic is also important. Could be something at the level of the soul(s) I guess