Magic is always a challenge for a fantasy writer, I think. Not actually writing about magic, per se, but dealing with your readers’ expectations of What Magic Should Be. Magic is the element, I think, that makes fantasy into fantasy. I mean, what’s Urban Fantasy, at its core, if not The World We Know, but with Magic?
You have your baggage about magic, your readers have their baggage, and when the two sets don’t match, someone is going to get annoyed. Probably them, because they think You Are Wrong.
The thing I don’t get is, you can’t be wrong. What you can be, though, is inconsistent, and that’s a problem. It’s important to know your rules of magic. What I don’t think is necessary is to spell out Rules of Magic explicitly in the text, at least not right off the bat. I have had beta readers who say I need to, though. I’ve also had a reader say I wasn’t playing fair because my character used magic on page 2, and I hadn’t established that yet. I’m not sure how much sooner I could have established it unless I just plain opened with it.
On top of that, I think people have expectations of What Having Magic means, which I find fascinating, but what it means depends on how common it is. On one side, you have things like The Belgariad, where magic is so rare that most people think it’s just stuff of legend and myth. On the other side, you have something like Steven Brust’s Vlad series, where magic is so common that even getting killed is mostly only an annoying inconvenience, since a friend of yours will have to pay for a resurrection spell. So you’ll again have trouble if people expect magic to be more like Vlad, but you write it like Belgariad.
I don’t necessarily have a solution, beyond, “It’s my world, I know how magic works, don’t tell me I’m wrong”. But I think it’s important to be aware of these things.