It's gotten too broken, repetitive, and demanding. Karn, being unbeatable, was the straw that broke the camel's back. He's WAY too overpowered for any Marvel character to stand a chance regardless of equipment or training. On top of that, I'll never get any SO heroes because it takes an eternity to get anything! I'd probably have to be an Alicorn to live long enough to get absolutely everything in M: AA. And I don't even know half the cast. Playdom also refuses to add any Power Rangers or ponies to make things manageable.
I'm also getting an unknown job soon, so I can't afford to waste any more time with repetitive grinding. This may get me fired. All the time spent grinding in M: AA just to get one hero could be an entire season of Super Sentai. And now I feel less comfortable watching Marvel movies because they will only trigger flashbacks to the worst parts of this game.
M: AA is also hacked a lot, especially by desperate players. Someday the hackers will destroy it, undoing all the players' hard work in one fell swoop.
Nowadays, I would prefer a full action experience over any number of turn-based battles. Super Mario World, Mega Man, Sonic The Hedgehog, and Super Smash Bros all give the player far more control than M: AA ever will.
This also means I can no longer fic. But I would still totally play My Little Pony / Power Rangers / Mega Man: Robot Master Alliance if they had none of the problems that M: AA had.