Gotta admit, I'm very surprised with how much I'm enjoying Black Ops 2. Speaking specifically of single player.
For me, the best Call of Duty campaigns were the first and the second, and Modern Warfare. I remember way back when the first was announced, and the "big thing" for the time was how many AI/NPC it was pushing in a single scene. The whole "horrors of WW2" thing, waves of people rushing against a bunkered down enemy, etc. I think that's why so many people, like myself, have fond memories of the Battle for Stalingrad sequence from the original game. Coming off that boat and pushing through the square, seemingly hundreds of characters on the screen rushing the enemy, getting mowed down. Had a great sense of pure chaos and carnage, like you're just one individual in the middle of two massive forces throwing themselves against each other in a wide battlefield.
As much as I enjoyed Modern Warfare, that marked the series turn from the above philosophy to more personal, tiny encounters and stage design. But I still enjoyed it, probably for the change in setting. I didn't mind Black Ops, but I loathed Modern Warfare 2 and Modern Warfare 3. I though World at War was kinda shitty too. MW3 I downright fucking hated, and I standby my belief that as a single player game it has abysmal level design, shitty pacing, and some of the worst encounters in the series. It's supremely boring, dull, and devoid of fun. MW2 wasn't far behind.
Black Ops 2, however, in many respects goes back to the philosophy of the original. Many of the battles take place in much, much larger battlefields than anything the series has seen since Call of Duty 2. There's a great sense of scale to the battles, lots of guys fighting on both sides, and the game constantly pushes you forward. Less squatting to grind against waves of forces, and instead a good momentum that keeps you moving through the battlegrounds. You cover a lot of ground in a single level. And even then, many of the encounters, though trademark Call of Duty scripted, are far less bottlenecked and overscripted than they usually are. If you need to take out a tank with explosives, or you need to push through enemy forces to an entry point, you're given quite a bit of flexibility to just how you're going to pull it off. Like, in the tank example, it's less "there's a mortar placed in a specific location, get to it, use it, and bomb this tank in a specific location", and more "here's a rocket launcher with anti-air homing capabilities, there's tanks and aircraft over in that general area moving about, now go destroy them". It's still scripted, but it puts more control in the hands of the player in how they're going to experience and engage in every encounter.
It's also completely utterly insane in themes and set pieces to the point where it's not like any Call of Duty. Not like the original gritty World War 2. Not like the wanky Modern Warfares. Not like the conspiracy mumbo jumbo Black Ops. Ironically, given the above, it's less "Call of Duty" than any Call of Duty, in themes at the very least.
I mean, it's still got some problems and rough portions I'm not overly impressed with, but it's clearly taken some steps away from the series' current formula to try a few different things, particularly in scope and level pacing. There's some other ideas thrown in there, consistently too, that lead me to believe Treyarch was legitimately trying to give the franchises a much needed kick up the arse.