Ace Combat 4, 5, Zero, and Six were the "Golden Age" of the series, combining very dramatic (somtimes a little overdone, but I'm okay with that
) plots with intuitive controls that required a fair amount of skill without being inaccessible, and at the same time really giving you a sensation of flying.
The problems with Assault Horizon are twofold. First, in the gameplay area, it replaces the relatively simple gameplay mechanics with (as mentioned above) a weird mix of Quick-time events and rail-shooter sequences. The results are visually impressive, but they take the skill out of the dogfights. All you have to do is time the button sequences right, and the CPU does the work for you. Not very satisfying. (Complex mechanics =/= complex strategy and simple rules =/= simple strategy. Try Go if you don't believe me: http://en.wikipedia.org/wiki/Go_%28game%29
Second, it's complete reboot of the old continuity, and the plot discards the dramatic, philosophical writing and deep characters for what's basically Battlefield 3 with planes... and very shallow characters indeed.
Finally, I actually kinda like the Apache sections (there are actually less QTEs), but the door-gunner sections and the AC-130 mission combined the strategy of a whack-a-mole game with stupidly high difficulty (at least with the AC-130)