On the one hand, since the mirror is being seen as a "neutral" thing, it could be considered as a "reality" of Tolkien's sub-creation; or even as a gift of prophecy. On the other hand, it smacks of divination. Sounds like a poorly chosen device to me. (Don't give any more away! ;-).
Also, I wish that Tolkien had given Gandalf a different title, other than "wizard," even though "guardian angel-like" is how I would have described Gandalf from reading the first 50 pages. It would certainly be misleading for children.
How about Istar (Wise One)?
In his great fantasy epic, The Lord of the Rings, Tolkien also portrays magic as deception. Supernatural powers that do not rightly belong to man are repeatedly shown as having a corrupting influence on man. While it is true that Gandalf, one of the central characters, is called a "wizard" throughout, he is not in fact a classical sorcerer. Tolkien maintains that Gandalf is rather a kind of moral guardian, similar to guardian angels but more incarnate. (Letters of J.R.R. Tolkien, edited by Humphrey Carpenter and Christopher Tolkien, Boston, Houghton Mifflin, 1981) In letters 155, 156 and 228 he explains his depiction of matter and spirit, and the distinction between good magic and evil magic. In essence Tolkien's "good magic" is not in fact what we think of as magic in the real world. Gandalf's task is primarily to advise, instruct, and arouse to resistance the minds and hearts of those threatened by Sauron, the Dark Lord of this saga. Gandalf does not do the work for them; they must use their natural gifts-and in this we see an image of grace building on nature, never overwhelming nature or replacing it. Gandalf's gifts are used sparingly, and then only so far as they assist the other creatures in the exercise of their free will and their moral choices.
This is a very subtle and clever way for the author to make his point by slightly changing what is actually written by Tolkein.
First, understand this: In Middle-Earth magic is not considered evil by it's own existance. Since Tolkien "sub-created" Middle-Earth, he can make the rules of Middle-Earth. Some other rules of Middle-Earth include very little religious activity (only one scene which shows any form of ceremony and that is in the third book), a poly-Valaristic governship of Middle-Earth (Iluvatar, the Creator, created the Valar, or arch-angels, and gave them dominion over creation. Iluvatar rarely directly interacts with his creation.), and "evolution" (The hobbits themselves appear to have been a race of man, orcs are elves that Morgoth twisted, trolls are Ents that were twisted by Morgoth, Uruk-Hai are a cross breed of orcs and men created by Saruman, etc.). While Tolkien was a devout Catholic and a dedicated apologist for Christianity, these are the rules he created for Middle-Earth.
Second, Gandalf is a Wizard is the very traditional sense. He is a Maiar, lesser angel, rather than human, but he is in every way a wizard. The five Istari came to Middle-Earth to oppose Sauron, a Maiar that was devoted to Sauron, because the Valar had decided not to directly interfere with Middle-Earth anymore. Gandalf took on the physical body of an old man, although it was immortal like an elf's body. When he was at the doors to Moria (you may not be there yet, so I will try to be careful) he tried to cast every opening spell known to elves, men and orcs. Yep, he didn't use an internal power, or an inherent trait, he tried "spells" known to elves, men and orcs. Do elves have an inherent magic? Yes. But they also have spells. Do men have an inherent magic? No. But they also have spells.
Third, we turn to the magic of the elves. First, Elrond at Rivendell. Rivendell is a place where evil cannot come. When the dark riders attempt to cross the ford of the Bruinen, the river rises up at sweeps them away. Is this the power of the river? No, it is the power of Elrond, who holds one of the 3 rings of power given to the elves. By himself, he could not protect the Valley of Rivendell, but with the ring, he has the power to do so. The same with Galadriel and Lothlorien.
As we can see, Tolkien, as sub-creator, has created rules within his creation that would be witchcraft here in reality. What Rowlings has done is sub-create her world with rules that would be witchcraft in reality. But it's not reality, any more than Middle-Earth, or MacBeth, or 1001 Arabian Nights, or Cinderella.