YES the most obvious - Auto Frame Rate on the Apple TV 4 and 4K.
Surely that's the H U G E S T picture difference ever especially when this forum was full of the AV guys crying like crazy when the ATV didn't offer this originally! Watching iTunes film is as smooth as butter!
You'll also find that ATV has far better components, longevity and a better upscaling of 4K.
Once it has Dolby True HD and HDMI 2.1 there would be zero missing from it, although i'm sure the AV Disc people would still find fault with it. It's what they do.
If the Roku was a car, it would be a Ford Focus (in beige). The ATV would be that sexy, red Ferrari with the classiest driver ever that you can't afford but lust after

Enough said!
Nothing compares to the ATV and it's rude to do so IMO