What is it? The setting, the general plot, the creatures or people?
Personally, I like the fire lizards. Who wouldn't want a pet mini-dragon that communicates telepathically?
I've only read the "Master Harper of Pern", but from what I remember it was a rich and in-depth world. I remember my favorite feature being the dragonriders themselves, and how they bond with a dragon, and change their name.
I also like how they have Bardic/Arcane schools set up. =D
First i liked the setting, it is similar to Zimmer-Bradleys Darkover, you have people from an advanced civilization stranded on a backwater planet and they become feudal societies. Also the problems they face and their development over the centuries/milennia. The books are definetely a must read for fantasy fans.