This. I love consistency in the rules as laid out by the author. It's inconstancy that breaks my suspension of disbelief, and that applies to fantasy, scifi, or a story of any genre where the characters suddenly do something out of character (especially if it's obviously for the purpose of driving the plot). Iron Man 3 made my skin crawl as the rules constantly changed throughout the movie for the sole purpose of driving some aspect of each fight sequence. Or the ridiculousness that was Gravity, just so much of that movie... ugh, doesn't help to have a working knowledge of orbital mechanics. (movie, book, w/e the suspension of disbelief rules are the same regardless of media)

As an interesting aside, once something has rules, even if the rules involve some level of unpredictability, then that something ceases to be magic in the way described by Less Wrong. It can be studied and it can have useful predictive models built around it. The problem with magic is the "because magic" explanation. If you imagine a world with "magic" and are able to deconstruct the reason for some magical occurrence in that world according to a reliable predictive model then the explanation is no longer "magic" at all.

Another interesting aside, just think about how magical things in the modern world would seem to someone without the background knowledge needed to understand it. Is that box shoving electrons back and forth to flip binary switches allowing me to store, manipulate, and search the internet for information? More likely that box is hosting a malevolent spirit. The first explanation is just too absurd.