First of all, the dolls are the most lifelike/beautiful that I've seen from a selection of eighteen inch dolls. There are so many different dolls to choose some - lots of variety - which is pretty cool. There's a doll for everyone!
The curly blond-haired MAG looks totally different than Felicity who looks totally different than Kaya.
The stories/accessories/outfits of the historical characters are so... historical! You can really learn a lot! Plus, the books are very well-written (including the mysteries and the history mysteries).
The dolls are so photogenic...
Plus, you can do all sorts of fun stopmotion/live action video things with them.
(from the episode City of the Edge of Forever)
Mostly, though... their stories are amazing.
Live long and prosper!