Magical systems are part and parcel of the fantasy genre, or rather I should say, magic itself. The idea of a magical system that works on certain laws, however, seems to be a newer phenomenon. Such as how magic works in any given "world", who is able to use it, and clear explanations about what governs the magic and the consequences to using it, if there are any. Is it high magic or low magic? Is it based on some mysterious force or supernatural phenomenon or is it based on scientific principles (or pseudo-science would be more like it.)
There are things I like about this and things that I dislike. This is my issue with this phenomenon, while I like the idea of figuring out and explaining your magic system and showing that there are direct consequences that come from using it, I do not feel this is necessary at all in a fantasy story. Want to explain every detail in your system and how it works? Fine, do that. If it is done well it enhances the story but it isn't needed. In my opinion it brings fantasy closer to science fiction and as far as I'm concerned, the two, while sharing a few important characteristics, are markedly different. When I want "science", I read science fiction. I see no good reason to write fantasy and hide behind a science fiction cloak.
Magic is not science and I would go so far to say that it really has nothing whatever to do with science. The wonder and adventure in science fiction is partly due to the scientific or quasi-scientific ideas and technology in its stories. The wonder and adventure in fantasy is partly due to the mysterious and unexplained, basically its magic. Ever heard the saying: "Leave a bit of the mystery?" I think this quality is important in fantasy.
Unfortunately it seems to me that fantasy as a genre is suffering from an inferiority complex these days. So many see fantasy as the bottom-scraping dregs of all genres; childish, foolish and stupid. Are we fantasy readers so embarrassed by the tropes and concepts in fantasy that we would like fantasy stories to "pass" as science fiction instead? I've read one too many forum threads of fantasy readers complaining about magic systems not being explained or not making rational sense and while it might be a nice layer upon the story to have a well explained, logical magical system not all fantasy stories require or this nor should they have this. For a story of high fantasy, it takes away any mythical feel or quality to the story, in my opinion. This doesn't mean that I think that there should not be consequences to the use of magic in a story. Take for instance Tolkien's The Lord of the Rings. In it, magic is more of an omnipresent, mysterious thing. There is no actual magical system that is explained or laid out plainly. Magic is infused everywhere in Middle-Earth. It is also used by many beings in that world, great and small, such as the elves who have their own relationship with magical craft, the skill of the dwarves who have fashioned their own magical objects and especially the Maiar and the Valar. Tolkien's magic is wonderment and subtlety. In fact, it works more like divine power than like magic and when coming from beings like the maiar and the valar who have remained in union with Illuvatar, it probably is divine power. Certain objects are infused with magic. Some with evil magic and others, while not intrinsically evil can be misleading and deceptive when used and therein lies their danger. Objects such as Galadriel's mirror, the Palantirs, the rings of power and most of all, and full of evil, the One Ring. There are terrible consequences to using some of these things but Tolkien did not have to write out any magical system to get this idea across to readers.
Perhaps I'm just being a cantankerous old curmudgeon. After all Brandon Sanderson has great magical systems in his books - or so I hear (I'm getting ready to read the Alloy of Law) and like I've said before, if done well it can be a good thing. But not all fantasy stories need it. Please, for the love of the Valar, let some of the mystery of magic remain.