Megan McArdle sums up one of my big problems with the world of Harry Potter in terms much better than I could:

Why are books about magic so exciting? The lure is almost tautological: magic is compelling because it allows us to imagine doing the things we cannot ordinarily do. Sure, romance novels may let you envision a world full of hot, sensitive men who want to cosy up to your wounded inner child, and do the dishes afterwards. But only in magic books can you make them disappear and reappear at will.

But this actually presents a problem for authors. If magic is too powerful then the characters will be omnipotent gods, and there won't be a plot. Magic must have rules and limits in order to leave the author enough room to tell a story. In economic terms, there must be scarcity: magical power must be a finite resource.

JK Rowling is not, to put it mildly, known for her seamless plotting or the gripping realism of her characters, most of whom spend the latter books pointlessly withholding information from each other that, if shared, would end the installment somewhere around page ten. But for me, there is another problem with the books, one that has kept me from looking forward to the seventh volume as keenly as I might. I am an economics reporter, and the books are chock full of terrible economics.

There are two ways, I think, that one can present magic: as something that can be done, but only at a price; or as a mysterious force that is poorly understood. So in Orson Scott Card's Hart's Hope, women who perform magic must pay the price in blood, their own or that of others.

Those prices provide the scarcity needed to drive the plot forward.

That's just the first bit. Go read the whole thing. She's right, and I just never put my finger exactly on it, but she has.