I've been wondering that same thing but haven't been able to find anything on Apple.com or through googling about it.
I don't know about that. Siri was a 4s-only feature of iOS 5. And I have a funny feeling the 240fps video mode on the 6 won't be available to the 5s - but I'd love to be proven wrong there.
Hardware specific features like Apple Pay, which requires the new NFC hardware built into the 6, obviously won't work on older phones - just like Touch ID on the 5s was specific to that device. But when it comes to all the software features of iOS 8 I would love to know what the rest of us do and do not get to enjoy