1. Mood Detection was an excellent way to automatically update status.
2. Being able to Auto-Tag a Photo based on past knowledge of a person (whether celebrity or after training on general users) was a handy secondary feature, but admittedly this might be considered the most useful for some.
3. Last but not least, I always thought that being able to suggest similar photos (for example of products not just people) would have been a useful application in comparison shopping, but couldn't ever get that working well with Face.com's API.