Megan McArdle takes issue with the economics of Harry Potter:
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.
But there have to be generally accepted rules. Characters can't get out of the predicament the author is sick of by having the car suddenly start running on sand. Similarly, if your characters will be using magic, they must do so by some generally believable system.
Yet in the Potter books, the costs and limits are too often arbitrary. A patronus charm, for example, is awfully difficult - until Rowling wants a stirring scene in which Harry pulls together an intrepid band of students to Fight the Power, whereupon it becomes simple enough to be taught by an inexperienced fifteen year old.
The arbitrary ham fist of Ms Rowling is everywhere too evident - changing the rules, and then making the characters tap dance, like marionettes, to distract you from the enormous potholes in the plot.
A good economist always tries to explain the world around her, and Megan is a very good economist. But even practitioners of the dismal science have to loosen up a bit sometimes. We make our living by selecting the constraints we would be reasonable to impose on our models in order to predict real world behaviour. In art and literature, any such constraints are redundant. The story simply has to be entertaining, or enlightening: realism, even internally, will sometimes be entertaining and sometimes not.
In Greek mythology, the Gods are all-powerful, yet this does not affect the quality of the stories the least. Once you accept 1+1=3, what's the point of pretending 2+2 will always equal 4? What's the difference if the Deus ex machina (από μηχανής θεός) enters the stage not as an independent actor but from within the main characters?
Or look at surrealism: where does a preoccupation with scarce resources, or the laws of the universe for that matter, fit in? Your world doesn't have to be as crazy as that; but if you want to cut it as a fiction writer make sure your micro books stay safely out of reach, locked in a cupboard far, far away.