Many people, on this, the old, and other boards, say that a good magic system is important to them in works of fantasy. Why is that, and what is meant by a "good" magic system? Is it in terms of complexity and internal logic, or simply novelty? In what way does this improve a story?

I mostly don't care. I tend to skip page-long exposition about a particular author's take on magic, thinking it's much like Faster than Light drives in science fiction; it's there, it serves the story, fine, I accept it. For it to be more interesting to me, these concepts will have to reflect some actual issues, by way of reflecting the themes of the novel more directly, or otherwise try to elucidate som point about human nature or the nature of story itself, as Hal Duncan did with The Book of All Hours, or Goodkind with Sword of Truth (no matter what your opinions of Goodkind, it cannot be denied that this is what he attempts to do). Erikson's magic is essential in that it directly shapes the narrative, but other than that I could stand to read less about the particulars of the Warrens.

Many author's organize their magic into pseudo-scientific systems, something I think many of them pick up from a background in Dungeons & Dragons and similar media. It makes sense in games, but as a litterary device, magic has more in common with the subconscious workings of the mind than theories of gravity and electromagnetism. Magic is by definition unscientific.

I'm not saying a marriage of science and magic shouldn't be attempted. It is done to great effect in steampunk and other works commenting on the relationship between man and technology.
But in my mind, a complicated magic system for the sake of itself seems like needless masturbation.

You thoughts?