Inspired by several lengthy debates on G+.
What I want from magic.
Magic Is Part of Me
In my ideal magic system, magic is an inherent part of the person wielding it. It's an attribute just like Strength, or Intelligence, or Speed. You can increase it by working out, you can learn ways to maximize your inherent abilities, but you're not going to go much beyond what your natural limits are unless you have help. It's part of someone's DNA basically.
Magic performs as part of a set of "natural" laws. The magic user might not know all of those laws, the reader might not know all of them, but they exist.
The first manifestation of the user's magic is almost invariably by accident. Something happens that the magic user can't explain or control. After further training and guidance they are able to learn how to control the maic.
The Wheel of Time, and The Wise Man's fear are both great examples of this.
Behavior isn't important
I don't really care how the magic acts. Flavor isn't important to me, as long as it fits the setting. The strange and mysterious magic of The Lord of the Rings is as exciting to me as the not so mysterious magic of The Wheel of Time.
Magic Has a Cost
Magic is dangerous. It should be treated with respect. Either you can go mad from the wielding of too much magic, or it can burn you out completely (e.g. Wheel of Time). When you've used magic you should be exhausted both mentally and physically. I wouldn't expect someone to run a marathon without needing extensive rest, they shouldn't be able to wield magic without doing the same.
Magic is Rare
Magic users are a rare breed. In my perfect world a person who can use magic is less than 5% of the population and someone who's actually powerful at it is less than 1% of the population. This brings along it's own set of consequences for the magic user.
Magic isn't Boring
I don't particularly care if magic items are rare or not. I do care that they're not boring. They should be useful, and they should have interesting histories. Sting isn't a great blade because it's a +2 sword with a +3 against it's chosen enemy orcs (as well as detection). Sting is interesting because of the history of the blade and how it's used. All magic items should be this way.