To really appreciate the movie you have to be familiar with Japanese storytelling and mythology.
The movie showed influence from many, many fairy tales and myths in Japanese culture, and the ending is typical of how the stories wind up.
If you compare it to current standards in American storytelling where movies need to either a) end in a completely opposite direction than expected, or 2) have a huge, gala climax, then yes, you'll probably be disappointed. The Japanese are more fans of subtlety than anything else.