I bought AoD when it came out and consider it to be the best incarnation of HoI2 so far.
It is certainly not perfect, but they made a LOT of excellent changes that gives the player a lot more strategical and tactical choise. I haven't played it in a while, so let me see what I can remember:
(1) New supply system:
Each unit now has a supply stockpile of its own (iirc they can story 2 month of their regular supply usage). Being idle usually replenishes supplies (depending on effective infra iirc), being in combat will deplete the stockpile quickly, so you can't press 24/7 with your units anymore.
This also makes encirclements a lot more interesting, because you have to starve the enemy before they suffer lack of supplies. Will you attack to deplete the encircled units faster but risk flank attacks from the enemy to free them? Can you intercept enemy transport planes that try to resupply them?
Ships also have a supply pool, which means they actually have to return to base from time to time to resupply, so controlling naval bases and ocean provinces is WAY more important than in vanilla. For example, GER has now a real interest in controlling the french and norwegian ports for their submarines in the atlantic. Same for USA/JAP in the Pacific - whoever controls the islands gains a significant advantage over the enemies.
(2) Artillery bombardment:
There are a few additional combat options and bombardment is one of them. This uses insane amounts of supply and deals way less damage to the enemy than regular combat, but the enemy units cannot effectively retaliate against you and damage the target province. Allows some sort of siege warfare (like in RL Leningrad). There is also a siege artillery brigade for these situations: slowing your unit down to a crawl, but giving very high ART attack.
(3) National Ideas:
As in EU3, every nation now has 3 national ideas (political, diplomatic and economic, iirc), selectable NIs depend on your ideology (USA has different ones than GER has different ones than SOV). These grant moderate benefits and allow to adapt your country a bit more to your strategy.
(4) Changing leaders, ministers and NIs takes time:
No more teleportation of leaders and instant-benefits from ministers. Changes take a few weeks (leaders) to months (ministers, NIs).
(5) Victory or Walhalla and Scorched Earth:
Land units can now be forced to hold their position at all cost. That means the unit will continue to fight if all ORG is gone, but usually lose STR quickly (and, iirc, cause some dissent). Has saved me a few times, I remember Manstein using VoW to hold open the last escape path of a large soviet encirclement, saving 20 divisions (at the loss of 3). Ships have a similiar option that will force an engagement and prevent them from trying to auto-escape combat.
Scorched Earth means that your unit starts to destroy it's province once combat starts (more units scorch faster). This usually means that your enemy has to repair more stuff - which is quite important because AOD added a new (and really good) repair system (will explain below).
(6) More tech team slots and new research cost system:
You can now have up to 10 research teams active at a time, but research at 100% speed does now cost a LOT more money (you can freely adjust your research %), thus vastly increasing your IC demand for consumer goods. So gaining a tech advantage now requires way more IC, making it a choise instead of a nowbrainer. This is very important for the USA, since it has to split it's early game IC between building stuff and researching.
(7) More units and brigades:
The game adds a lot of new units and brigades - assault guns, infantry tanks, naval brigades like sea mines and spotter planes, heavy submarines, etc...
(8) More techs:
Not too many, but there are a few.
(9) Infra system and new buidlings:
Infra can now be improved to up to 200% and affects ressource income, so you actually have a way to spend your IC to improve ressource income. There are also a few new buildings like synthetic oil and rubber plants that are especially important for GER or ITA and allow a few new strategies.
(10) Air warfare and the repair system:
AoD is the only HoI incarnation that actually models tactical and strategical air warfare in a decent way. Actual air-to-air combat didn't change too much, but strategic and tactical bombing is vastly different. For example, you can now bomb airfields to not only damage the airfield itself - but also all planes stationed there (like the german Luftwaffe did in Poland and the Soviet Union).
Strategic warfare is, by far, one of the best features from AoD. First of all, there is only a very small amount of free auto-repair for provinces, so you have to pay IC for repairs (and you can, btw, prioritize provinces for that). This means that strategic bombers are actually useful if you manage to gain local air superioty. If you can damage their IC provinces (or even better: enemy synthetic material factories), you will force him to spend his precious IC to repair instead of building new stuff. I actually played an USA game where I invested most of my IC into strategic bombers and bombed Japan down to a constant 10 IC and let it bleed out. Same for germany afterwards). Air missions are a lot better to coordinate, too, because you can now assign regions (and even countries iirc?) as targets.
At the same time, AAA is actually useful. Not only does it fire at planes that fly above it's province, but it also receives a HUGE bonus from radar, making static AAA a valid strategy to protect your important provinces. I remember an ENG game where I had fortified Hong Kong with 10 AAA + 10 radar and dealt 30-50% damage to a Japanese airfleet in a single attack run. So while strategic airwar is much more dangerous, you also have a lot more ways to effectively protect yourself.
(11) Production:
You have more control over your production. For example, you can change the amount of units from a serial run you started earlier, you can pause or continue serial runs (and pay a small upkeep IC to save your gearing bonus), you can prioritize units to pay 2 or 3 times the IC demand and get a decent build speed bonus (25%, iirc?). You can also upgrade units to the newest model, saving some of your gearing bonus.
(12) Casualty Statistics:
The game keeps track of destroyed units and calculates losses for each country, giving you a completly overview in the ledger.
There is probably more, but thats the most important stuff I can think about atm.
