Maybe it's because I'm such a gigantic book-nerd...but I tend to enjoy the idea of magic that follows certain rules. Since I read a LOT I've come to recognize some key characteristics that make me enjoy magic, I'll be employing these in my RPS.

1. Magic can't spring from nothing, and must have consequences.
This is a fairly common theme among the best authors who use magic. The mage will eventually run out, and their skill as a mage depends not only on their magical knowledge, but on their ability to apply the least amount of power to the greatest effect. In addition, using up all of the currently available magic will have consequences, and dire ones. A mage who drains their stores completely risks unconsciousness, long term reduction of their power-level, long-term health effects (maiming, injuries, loss of magic entirely, ect), and also death.
When magic is unending in it's use I find myself getting bored. Sure, Lord Thus-and-so killed three dragons and an entire orc horde by himself, but if he does it without breaking a sweat and seemingly without any real cost to himself who really cares?

2. People have different skills, talents, strengths, and weaknesses. Mages should to.
I know everyone here is familiar with Mary Sue's and why they're no fun...this is sort of an extension of that. To me, if all mages have the exact same level of power and all that varies is how clever you are...wouldn't a small band of 'good' mages easily be able to wipe out a singular 'bad guy'? Boring.
Some mages should be weak, like lighting a candle once a week levels of power. This makes it just that much more fun to apply your meager power to good effect. Using your intellect to boost what you're capable of. On the flip side, why would it be fun or exciting to be Superman if else was ALSO Superman/woman. It wouldn't. You need balance to make it enjoyable and add another level of realism. Different strengths and weaknesses make playing fun, if everyone can do the super-cool unlocking spell to open the dungeon, and the fire ball spell to kill the ogre, and the hover spell to cross the moat...why did it take 6 of you to get the treasure?

3. Do mages spring from their mother's loins as fully capable Masters?
You need to learn to walk, run, jump, speak, ect. IMO incorporating a learning curve into a magical RP adds a lot more options and excitement. If you join up and immediately start blasting away with your awesome wizard-bolts...how do you move up from that? Again, boring.
Mages should be able to learn tricks from fighting their opponents, to push their own boundaries - magical, mental, physical and otherwise.

4. What greater cost does magic use have?
If you use a system of magic where the power comes from outside of the mage where does it come from? How does that drain effect the surroundings? Can magic backlash like electricity? What does that cause? What does a complete drain on all the magic of an area do? What does letting loose a gigantic storm of power (as in a mage-battle) cause on the surroundings? How do the lowly townspeople feel about you slurping the power out of their crops in order to kill the Evil King Tax-a-lot?
Again, considering these points adds another level of depth and realism to the game.

*shrugs* just my thoughts :) I hope I was clear enough, lol