A few things I've found that are interesting.
- According to Apple's Website, the 6s will not only have the 3D Touch, but it will provide subtle haptic feedback along with the force touch. This means it will work similar to the trackpad on the new Macbooks.
- Dynamic Wallpapers. At the same link above, there is a section that shows iOS 9 will include dynamic wallpapers. Live Pictures can be used as Wallpapers. They mentioned this would be in the Watch OS as an option, but I don't recal them mentioning it's availability on the 6s. Hopefully battery isn't hit too hard here.
- Faster LTE and Faster Wifi - Apple's Website Here
- 4K Video recording isn't QUITE 4K @ (3840 by 2160). Also "4K" is only available at 30fps. So don't count on taking that 4K Action or Sports video - blur might be somewhat obvious. 1080p is available at 60fps.