While I've only done basic animations in Spine thus far, if I had to list features, it would be these:
- Motion capture / rotoscoping and animation transfer
(At about 2:10 in this video)
- Cloth dynamics / weighted physics
Auto walk cycles / walk motors
Path animation
Of all these though, I think the motion capture is very impressive. In addition, check out the videos on cloth/hair dynamics. It's very interesting how they are using weights and physics to achieve it.
There is also a nice animated Tiger which no doubt is great for marketing:
There are a bunch of YouTube videos of the app here:
https://www.youtube.com/channel/UCPqZjHnq8U1hFehOUqEmgyQ
It's obvious the people/person who made this put a lot of effort into it, observing that Spine is it's only real competition.