There's been a lot of discussion about monster aggressiveness (and whether the monsters attack the human player more than the AI), but I think a lot of the problem comes from having no way of understanding the intention of the monsters.
They stand there like they're in a petting zoo for a hundred turns and then something clicks and they march half-way across the map to attack a city. Why? Nobody knows.
Well, the world is a harsh place.
What makes it game-breaking is that there's no tip-off to this behavior. The player does not get a warning or even a hint that this monster isn't just patrolling, but is actually making a bee-line for your capital.
Monster intention is useful information that the game is hiding from the player.