Ditch tons of AW people here. As said Filth is here now and iko and several just came over who used to be in the Mighty Fluff.
I was and 3747 in AW .. with the Nightmares VMF-898. They are the Nightmares VMF-101 here.
You will find several AW squads scattered about .. NightHawks, CAF, Flying Tigers AVG, DAMNED, etc., etc.
As for the differences. Well there is a lot and will take you a bit of time to get used to them but after a few weeks it is no big deal.
FM:
Flight Model is different but when I came over from AW FR I did not find it a massive change. AH2 probably a bit more of change since it models 8 points of contact/force/whatever you call it enacting on the plane as it flies. Unlike AW AH1/AH2 the planes do not act as they fly on rails and several of them actually suffer from engine torque pulling them to a specific side.
You can fly with combat trim on or you can turn it off and manually trim your plane. Didn't have trim in AW.
Probably the stall characteristics of the plane will take a bit of getting used too. Several of the planes are real beast when you get slow and push the envelope. F4Us and 190s tend to roll and flip on their backs for example.
Could right alot more .. but think this will be the easiest to adapt too.
GM:
Gunnery model is a whole different ball of wax here. The hit shell / area of a plane is very tight on the exact shape of the plane itself. It got a lot tighter from AH1 to AH2. Not sure if is now exactly the shape of the plane or not but is very tight (small).
Many people have found that they have to get within 350 yards to actually hit enemy planes. Very big difference from AW long range sniping. Couple this with having to set gun convergence and you really have something different. Basically you have to select convergence points for your guns. Which is the point where all the bullets from your various guns intersect. Hit an enemy closer or farther out from that convergence point and your grouping starts to spread out .. making it harder to take down an enemy plane since you are not putting all bullets on one spot.
Also several of the guns are not as accurate as others .. 20 mm Hispanos are very accurate and produce tight groupings. Several of the German cannons are not very accurate and don't produce tight groupings (meaning you need to get closer and open up). Also different guns have different rates of fire and their shells drop off differently.
In LA7s you need to get close to have a decent chance of hitting for most people.
DM:
They just redid the damage model and damage applies more exactly to where your bullets hit. Meaning you can shoot out engines, shoot off wings, etc.
Also it makes it harder for snap shots .. if you hit the enemy outside of your convergence range you might put say two rounds on a wing, two round on the tail, and miss with two rounds. The damage is applied to where is hit .. which makes it more important to get a good hit and keep a burst on target for a bit. Or several bursts.
Also there is more types of damage available in AH2 that in AW. You can shoot out rudder, elevators, aierlons, wing tip, wing, oil, engine, fuel tank(s), specific guns, and wound the pilot.
Obviously there is a lot more difference but it would take a bit too write. Some planes have multiple fuel tanks, you have have drop tanks, rockets, different bombs (sizes), bombers can be upped in formations of 3 and the bomb model is massively different here from AW. etc., etc.