This is a very good propostition of a thought exercise, which I like to do myself to put the current slew of next gen games into perspective.
Some people get upset about the vague classification of "not trully next-gen" or "generational leap" but most kind of agree with the sentiment and understand the idea thouch each probably has a slightly different understanding of the concept.
I see a true next-gen game as one that do most of these:
-Ambitious game design, through large amounts of graphical and simulational detail or through huge scope, or a combination of both, to a degree that would be though extremelly hard to achieve on the previous gen (I think AC, GTAIV, and Oblivion are good exemples here)
-PS4 and One are gonna have a hard time making that game after GTAV raising the bar as high as it had, but so did san andreas on PS2...
-Innovative or Highly improved production style / paradime changes. Last gen, producing all assets at absurd poly-counts and down-sizing them to normal-mapped lower poly meshes was a huge game changer. (Doom3 gave an early taste of what a game fully built that way could look like, Gears refined that)
-For this gen, the big idea is physically based shading and lighting, and use of robust techniches to scan and photograph real world parameters, here MGS5 is the early taste this time with its pretty avant-garde FOX engine and production practices, The Order might be the new Gears on that aspect.
-Innovative rendering algorithms: The impressive stuff last gen were, besides universal use of pixel shaders, the more common use of advanced post processsing and HDR, defferred lighting and propper dynamic shadowing. Killzone 2 was the big showcaser of those three all working in tandem and how much that added to mood and feeling.
-This gen, the initial popular new stuff seems to be better lighting models, vastly better post, Screenspace Reflections, GI and directional occlusion, and better shading of translucents. Note that all these are evolutionary steps ahead of stuff already being done on ps360, so there hasn't been any Ground Breaking new algo yet (Voxel Cone Tracing could have been it, if it was more feaseble) But Capcon's Deep Down, might be the one to take the crown on that aspect with its volumetric fire and dynamic liquids.
-Physics bitch: be it through numbers, or fidelity, or both. Half Life 2, though a sort of transitional game, was, for some time, sort of the informal benchmark of in-game real time physics done right.
-Sheer Numbers. Many cars, Many NPCs, Large environments, those don't revolutionise anything, but can still impress. Last gen, that was Dead Rising.
-Foward thinking Gameplay: Though we forget this, but ps360 changed the overall gameplay style significantly, mostly seeking a more personal experience. That includes the broader use of the analog stick for tricks and dribling in sport games where Skate diferentiated itself, the popluarisation of cover based 3rd person shooting where Gears did shine once again, or the CODsation of first person shooters, (precision aiming, regenerative health, highs sensitivity and other gameplay conventions)
Other honarable last gen games that trully felt a big step above ps2 era: Uncharted, Fight Night (though very limited in scope), MotorSport (largish numbers + largish scope + abundantish physics etc) COD4, Bioshock (mostly for the level of detail and fidelity of its environments and high production values, but not really a big technical showcase.
I still remember countless reports of YOLD from playing that game near its launch.
