Too many rules? Not enough rules? How should it work in the world you've designed?
Jamie Schulz said this on io9 recently:
"Good magic systems have a framework, a set of limitations that is fairly clear to the reader, such that magic doesn't run the risk of blowing the internal logic of the story to Hell."
which I thought summed it up quite nicely.
I don't have too many rules - for one thing, I'd never remember them - but on the other hand I don't want a character to have a "get out of jail free" card whenever they need one. So I have five sorts of magic users, roughly speaking - they can manipulate one of the four Elements of Earth, Fire, Water or Air, or they can influence human (or animal) minds. So they can be powerful, but only in restricted ways - a Fire user can't float in mid-air like an Air user, for instance. I use bastardised Welsh terms for them, because I'm not a Welsh speaker but I wanted a Welsh flavour to it, so my family of fire users are called tanwch - tan for fire, and -wch is a suffix which makes a word into a command, like when Arafwch is written on the roads, meaning Slow!
I purposely didn't have any healing magic in my world, because I really think it's a cheat. If a character is wounded or sick, they shouldn't be able to leap up and do important stuff immediately - they should lie down and suffer for as long as it takes.
But then I added unicorns to my world, and unicorn horns in the Middle Ages were traditionally used for neutralising poison, and healing. They're pretty rare - but it is a "get out of jail free" card, as long as the user of the unicorn horn is a virgin, of course.