These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Broken Aesop: The game seems to try for a Modern Warfare-esque War Is Hell-tone, which is all well and good... except that flying fighters is about the most glamourous thing you can do, and you aren't any fighter pilot, but one so legendarily good your enemies wet themselves and your allies start praising the lord when you show up at a battle. So, War Is Hell unless you're an awesome pilot, because then it rocks?
Broken Base: Gamers that bought Assault Horizon BEFORE picking up any of the other games will love it. Most long time fans hate it.
The biggest break point is the DFM system. Not only in this game, but arguably in the entire franchise. Supporters love it because it makes dog fights very spectacular and intense, plus it encourages to use it to people who in previous games just used missiles over and over since using the machine gun was way too hard for getting the same results. Detractors hate it because they think it destroys any sense of skill or challenge, and turns dog fights into glorified on-rails shooting segments, if not down straight Quick Time Events. The fact that some climatic boss fights forces you to use it doesn't help.
Stopping an ICBM launch by slipping under the missile's wake and accelerating upward after it, getting close enough to take damage from the thing's exhaust.
Engaging in a dogfight within the outer edges of a Category 5 hurricane.
Rescuing Guts when his ejection system is damaged by using his cannon to shoot off the canopy on Guts' Raptor.
D-Ray gets one himself, when, during the Moscow assault, enemy long-range SA Ms take out several helicopters and force all other choppers to duck down to avoid being targeted. Realizing that the mobile SAM teams will obliterate the fighters when they get back from refueling, D-Ray and his gunner Jack hunt down and destroy all the mobile SAM teams. And then, just as a final kicker, devastate the remaining rebel forces, essentially winning the battle single-handedly.
Complacent Gaming Syndrome: Expect Capital Conquest to be the game mode that's frequently played online, mostly due to the discovery of exploits that allow completion of the mode within minutes.
Game Breaker: Those Quick Air-to-Air missiles that were dreaded online and were the most broken missiles in the series? Yeah, they've been buffed back to AC 4 status, and when in Dogfight Mode, the reticle stays glued to the enemy plane you're targeting.
Double goes for the CFA-44 Nosferatu from Ace Combat 6, though the All Directional Multi-Missiles have taken somewhat of a Nerf.
Franchise Killer: Regardless how you feel about Assault Horizon, the game didn't sell as expected to the point where Project Aces simply ran out of money, forcing the creation of the Free to Play Ace Combat Infinity installment. As that game has been critically panned for its clunky online/FtP system, repetitive online/single player modes and lack of polish, it looks like this is the end of Ace Combat for good.
Love It or Hate It: Again, the DFM mode. People who hate it tend to be really, really vicious, though.
Mis-blamed: As stated in the trivia section, Jim DeFelice had plans for a story set in the Strangereal setting, but the decision to go into the real world was Project Aces'. Forum hopping on GameFAQs has people up in arms with regards to DeFelice's story for the game.
Considering the Strangereal nations are still based on modern or historical real-life countries, the concerns may or may not have remained legitimate had this game gone with the Strangereal setting. Besides, most of those criticisms lie anyway with the facts that the War Is Hell aspect of the series is much toned-down, the storyline spends too little time outside of Bishop's personal conflict (whereas those who already played any or all titles between 04 and 6 would likely expect much more than that), and that the player character is Bishop himself and therefore no longer stays anonymous in Assault Horizon.
Also, while there is significant vitriol towards Dogfight Mode mechanic, what is the the recommended method of ensuring the best kill probability in the PS2 trilogy? Getting right up behind an enemy plane, in gun range. Which is exactly the same as DFM, only manual.
They don't automatically home in on your target like DFM do, mind you.
DFM was not the problem. The negative feedback was virtually nonexistent when the trailers were announced. It was after the fact that DFM was actually also well-disguised cutscene/Quick-Time Action and mandatory in places to boot, then it became a problem. To compare, every older game, has you go up against an Ace squadron (or even a Final Boss a la Pixy) and ask you to shoot them down with your own skill. DFM is a tool, but the final boss fight against Markov is a forced on-rails segment that has to be done with DFM. If it were that DFM stayed as a tool instead of a glorified cutscene, then there will be much, much less negative reaction to it. After all, this is an expanded version of the one seen in Sky Crawlers: Innocent Aces and that was not received poorly.
Older Than They Think: Unlocking planes for use from the get-go without buying them from a shop isn't new to the series. That goes all the way back to Ace Combat 3.
Similarly, even Joint Assault wasn't the first game in the real world. The arcade games (Air Combat and Air Combat 22) took place in the real world, but they're so obscure only a few people even know those games exist.
Polished Port: The PC port adds a bonus content pack (extra planes, maps, skins and skills upgrades) and support for screen resolutions up to 1900x1200.
Scrappy Mechanic: The Close Range Assault mechanics are rather contentious, especially Dogfight mode.
Tainted by the Preview: The demo forcing you to use Close Range Assault to splash aces has not been warmly received.
Of course, this fight can be cheesed if you either a) bring an aircraft that has ECM, or b) learn to exit Dogfight Mode whenever you hear the lock-on beep and enter it again to make the missiles useless. Or you can do both to be sure.
That One Level: The AC-130 Level is damn near impossible in Ace Difficulty, because there's no way to prevent being shot at with SAM's from the ground and the SAM's are too fast to be shot. Couple that with the fact that you have to deal with two or three at a time, that two kill you and that you have to escort your allies, it's pretty hard.
The helicopter levels are often criticized for dragging on far too long and ending up dull and repetitive as a result.
They Changed It, Now It Sucks: Where to start... many AC fans are unhappy about the change in setting, the lack of superweapons, the introduction of Close Range Assault, the Regenerating Health and the addition of piloting different kinds of aircraft like helicopters and bombers, for starters.
HAWX was deemed inferior to Ace Combat as a franchise by AC fans, yet many old-time AC fans think that HAWX is superior to Assault Horizon. It's more of a "It's a completely different game with a completely different gameplay" than complaining about the changes, though.