The engine is tweaked for each iteration (Fun fact: Engines are commonly reused, as its more practical to just make revisions than design a new engine entirely from the ground up. See: Source Frostbite, etc.)
There have been small graphical improvements, however, keep in mind that their dedication to having the game operating at 60 fps severely limits what can be done in that department.
Better single player? You will have to be the judge of that. Find a friend that has it and borrow it, or rent it from a service like Redbox.
Companies that release AAA games that sell millions of copies generally update their engine to keep up with the times. But the IW engine is almost exactly like it was in 2007, with minor updates like improved lighting, and it shows; Capcom, Epic, Crytek, and Nintendo all have engines that make CoD look like crap.
In regards to the AI, it's entirely possible to have intelligent AI. F3AR had great AI, but the makers of CoD won't improve their own.
I've yet to play single player, but, as I've heard, A.I. most likely won't improve in general until we move into the next cycle of better-equipped consoles. There's only so much you can do with those 7 year old clunkers.
Apparently, you will show on the radar if you aren't moving over a certain speed, effectively nerfing Ghost, and forcing you to use a run & gun play style. Aside from that, I think the most significant change to the class system is the addition of Wildcards.
omfg im playing it now and yes im a girl but dont give a crap. the aiming system is great there is little to worry about someone getting easy kills just because the crosshairs help with aim assist and no helicopters shooting me down so easily since its not killstreaks its off of a point system.....kinda like mw3. that game already pissed me off i might go back to black ops to for fun sometimes but i will never go back and play mw3. The spawn system is alot better...........for now until you get them spawn faggot campers......and the way the maps are designed if you noticed its a little difficult to get flanked so easily all the time............so morgan webb and adam sessler i give it a 9 out of 10.
wow judge a person when you know them not everyone has to be stuck up, and because im not the average cod gamer, i can care less about what you said. i play a wide variety of games and cod is actually a little fun from time to time since i am in the Marines but like i said to the other person you have to know my bio or i have to sound like half of the pointless anime lovers on here for you not to take me serious so whatever. oh and btw i responded to a person who asked about the new black ops so unless you are on here to do so you are a waste of space on this particular forum as you are a waste of space on Earth. Grow up kid.