For me at least, the first part was an absolute work of art. Apart from the crazy, not always entirely comprehensible story, it was simply beautiful. I'll never forget the moment when I climbed through the vast landscapes for the first time and suddenly heard this phenomenal soundtrack.
The amount of multiplayer influence was also just right; you got the impression that someone had been here before, but you never met anyone. Wonderful.
Didn't expect a second part to be released, what a pleasant surprise!