Another quality topic by Amir0x.
A Next gen game for me, aside from graphic updates, would be something that couldn't be done before on the previous version of that system, something that uses the game's hardware or RAM to its full potential. Something extra needs to be done with those to flow more seamlessly into the gameplay and experience.
So for the PS4, anything that used the touchpad and gyros well. Killzone, while it looks fantastic, just used the touchpad as a second D-pad. Infamous just used the light bar and speaker for karmic alignment and a spray paint sound effect. I'm not too sure about how the lightbar can be used for a non-casual game (unless Project Morpheus really can use it for tracking), I liked Killzone's usage of it as a life meter king of thing, but it didnt really make it easy to see when I was playing it during the day time. Again, using Killzone, the audio logs from the game speaker was a cool idea, and it worked well in my opinion.
Not too sure about Xbox, as the only real hardware change aside from graphical updates was the Kinect and trigger updates, and its apparently harder to retrieve memory usage from (or just not as fast a PS4, but that's not the real point here). Kinect has significantly enough input delay to be used well as a game play mechanic (see Fighter Within), and with it being not included in the new model makes developing a game around it not too great an idea for a developer. If it really can detect a heart beat, using that as a sound method in a horror game could be cool. like if a jump scare gets you you can hear the music react to your heart beat. I really want to see a true next gen Xbox game, but i will need to wait a while for that.
Wii U just needs to use the gamepad well in a way that makes sense. A constant inventory screen, a cool input mechanic like W101, a map for a metroidvania, something useful that isn't just a black screen. No developer in their right mind made the second screen useless in a DS/3DS game, so why do it with the Wii U?
Tearaway on the vita was phenomenal as a next gen handheld game.
I know I'm preaching to the choir here, because of who the OP is, but whatever. it used all the Vita's controls and inputs in a meaningful way. no other vita game really did this that I can think of.
The 3DS cant really do anything to tell me its more than a graphically updated DS game, because the main mechanical update was introduction of a screen gimmick, and not a second analog stick. While it may look great, there really isn't any game that is ruined without it, and that's why the 2DS is a success. Unless a game is designed around the Circle Pad Pro, but I cant see that happening with the 2DS being as popular as it is. Which is a shame, because the second analog stick and 2 extra shoulder buttons would help the games have camera control, freeing up LR controls and add ZL a d ZR controls.