It really didn't though, even in the first game. That's just how most people chose to play. But the combat in AC1 was incredibly deep and varied, and allowed for enormous experimentation. But as with almost everything else in the game, most players were too impatient or indifferent to bother with the huge amount of experimentation that was possible. AC2's combat was dumbed down a bit, but then ACB and ACR built back up the huge experimentation toolkit. If combat really seemed that simple to you, it means you weren't taking advantage of everything that was available or possible.
In AC3, almost none of that huge library of techniques, weapons, and tools is available. It boils down to just a few techniques, but tries to make it seem varied by rolling in a bunch of varied animations instead.
Using the A button in different situations in addition to the trigger meant that you could choose to climb, drop, jump at specific times and contexts. But in AC3, just using the one button means that all of that simply "happens" and you lose the ability to choose exactly how you traverse the environment. Though oddly, they still make you push an extra button to leap/slide over things--which, ironically, is something that should have been just rolled into the trigger press. So by losing "sharpness," I mean that you lose one of the means of greater control and precision in your movement. As with combat, they've sacrificed player control and replaced it with automated animation.
But these are just minor nitpicks. However, they are emblematic of an overall design philosophy that favors automation (through an admittedly excellent animation system) over player control. That emphasis on automation over player control applies to how they designed missions, the frustrating stealth fail states, combat, traversal, etc. It's a series that has always been predicated on player freedom in gameplay (experimentation and exploration) and political freedom in its storylines, but there's very little of that left in AC3.
