For me, it's that they're a lot less like everything else out there. I love the Alanna books, and they're good quality, but they're very similar in their medieval-Europe-with-magic setting to almost all the other YA and non-YA fantasy out there.

Pierce said somewhere that the inspiration for the setting for the CoM came from her study of the Silk Road and the ancient Middle East trading centers where radically diverse cultures met and traded. Not too many books based on that sort of thing and add in a well-thought-out magical system, interesting cultures, and characters I love, and it's very easy for me to see why I love these books so much.