I think the two main selling points for me were the freedom to pretty much go where I liked when I liked and the system, which allowed me to develop my character in any way I wanted with scant regard for what class s/he technically was.
The atmosphere was really good. A LOT of work had been put into the background history of the world and it showed. I liked the attempt to fit all the races into the world, too, rather than the usual CRPG approach of 'well, they all live somewhere, I guess'. The world felt more real than most.
