Origins was really just a slightly different City with a (opinion) better story. It was done by a different studio, but to me that's not really an excuse. In fact, it was a good chance to change things up from Rocksteady's methods... then again, if they had the fans might have reacted strongly for nostalgia's sake so there's that. Sad situation for WB Games if you ask me.
I agree about City though. Was good, but I don't actually recall much that changed from Asylum aside from a bigger map. Now it seems to be the same for Knight. Personally I think the Batmobile adds nothing of value because it only seems to be used in the game for getting around faster and for shooting up tanks as you've said. And when I think of Batman... no, tanks aren't really what come to mind.
Probably won't be picking this one up if there aren't innovations to stealth and/or fighting. Preferably stealth. Otherwise, it just feels like the transition from ME2 to ME3 - an update more than anything else, except Knight doesn't have the pull of ME3's "continuing the story". That's not to say Knight won't be good; I'm just weary of the feeling of sameness between sequels.
Origins was almost exactly the same as Arkham City,except like you said, it had a very good story (I would say I actually liked Origins' story better than City's). Aye, it was done by a different studio, and was given the basic engine for the game. I thought "as long as the story will be good, why not?". I mean really, a different studio, sure they could've changed some things, but they weren't allowed to, and I don't know how the hardcore fans (like myself) would have reacted if they gave us something new, but it didn't work out well for the game. Even these shock gloves were a bit weird in my opinion, sure you needed it to activate elevators and stuff but it was just unnecessary besides, it made the fighting too damn easy. So I thought it's actually better they didn't change many things.
In Arkham City, the map was bigger, riddler informants were added, new combat moves like Bat-swarm, side missions, Catwoman as a playable character, lots of new gadgets, smoother combat system, and new moves for the stealth part. Nothing major, sure but still it felt new, at least to me.
Well what we've seen so far of Arkham Knight, there were some new things in stealth. Like the Fear takedowns, which look real badass I think. In combat, Batman can now use the environment to attack thugs, and the enemies themselves can counter his attacks. Unlike Arkham Origins, where only 1 type of enemy was able to counter his attacks, here most of them will be able to. You can throw batarangs while mid-air. You can even use your batmobile in combat on some moves. Changes were also made on the gliding system. About the batmobile, it's won't only be used for roaming around and taking down the Arkham Knight's tanks, but rescuing hostages, pulling down walls, plus the riddler challenges.
Edit - we really don't know about anything big in terms of changes except the batmobile. Rocksteady is really keeping the lid on this.
There was an article I read earlier saying that the rain will pour down slower on different surfaces, have different noise on surfaces. For example on Batman's cowl or skin, it will be slower than on metal parts of his armor.
Oh, and something fresh; this will be the first game in the series that will get a "Mature" rating, which sounds good, taking the story to a more mature level
http://www.gamespot.com/articles/batman-arkham-knight-gets-mature-rating/1100-6425487/