In my mind, "next gen gameplay" is gameplay that would not have been possible with previous gen hardware.
A good example from the previous gen would be Assassin's Creed, which, incidentally, many people were calling the "first real next-gen" game (or something to that effect) back when it was first announced. Why? Because Assassin's Creed would have been impossible to make on PS2/XBox, not just because of the graphics, but the overall complexity of the open world, animations, crowd systems, etc.
Second Son, on the other hand, could have easily been a PS3 game if they just toned down the graphics a bit. There is simply nothing in the game, apart from the graphics, that would not have been possible to implement on PS3. Hell, GTAV is arguably a much more complex game when it comes to gameplay mechanics, and it runs fine on PS3.