For the first game, most people never realized how many different attacks and defenses there were. You had to be an "experimental" player to learn how to use them. Things like guard-breaks, lunges, evasions, offensive combos, multi-enemy combos, grabs, aerial attacks, and so on. Then when you start messing with the different weapons, you end up with a whole slew of new timings and possibilities. Most people missed all of this (and a bunch of other amazing game features in AC1) because they insisted on just using the counter-kill over and over.
Then in AC2, on top of all the attacks in the first game, they introduced a whole mess of new weapon types, throwing knife combos, bullet combos, sand-throwing, disarms, smoke bombs, etc. And on top of that, they introduced different enemy types that responded differently to different attack strategies. But, like in the first game, most people who played the game never realized any of this because they stuck to the counter-kill for most of the game.
In ACB, you can still do all of that, but the group dynamics on the enemy AI doesn't allow for it. If you go into an offensive combo, another enemy will almost always interrupt you. Same goes for evasions, disarms, grabs, and so on. The group AI has become too aggressive to allow you to do anything other than counter-kill. The designers likely saw this, so they introduced the execution combos to try to make up for the loss of the other combat possibilities.
Even though you can still do all of those old attacks, they don't fit into the combat design anymore. Just about the only combat abilities you can viably use are counters, execution combos, guard-break stomp, sneak attacks, and assassin attacks. Now, clearly that's still quite a bit of variation available, but not nearly as much as there used to be.
But as I said, there's a lot of incredible new design approaches here--especially in the stealth aspects of the game. There's nothing more satisfying than sneaking into a well-fortified base surrounding a Borgia Tower, then finding and killing the captain undetected. Those sections are simply some of the best game design in any open-world game ever.
