Yeah, same problem here. I have the 2nd gen. Apple TV which only does 720p. But I understand 240 fps slo-mo on the iPhone 6 (Plus in my case) is 720p anyway, and I believe it is 30 fps. I actually can't get any video to Airplay off the iPhone 6. It either blacks out the screen or shows a still frame. And does not work with mirroring on or off. The mirroring of the phone itself works, I can see the UI and photos, etc. but video is a no-go so far. 😕
Hopefully this is another iOS8 bug that will get fixed, after all probably no one at Apple even tried it on a 2nd gen Apple TV.