This is a question I have wanted to ask for a long time. I can't quite recognize Wizard101's art-style because it's pretty unique compared to anything I have seen. It looks kind of like a blend of anime and those illustrations you see of fantasy novels (like Lord of the Rings or Harry Potter), with maybe some western computer animated movie style in there as well. If there is another game I can compare it's art style to (which there isn't really), it would probably be Castlevania: Symphony of the Night or the Legend of Zelda: Ocarina of Time. I really can't put my finger on what the art style is, I've never seen it before.
Actually, I have found something better to compare Wizard101 to not only in terms of art style, but also gameplay and premise. Golden Sun and Magic: the Gathering. Yet I still can't quite categorize it.