For me the basic gameplay is as close to perfection as I’ve ever played. Everything that you do in a moment to moment basis just connects with me like no other game.
First of all the combat: I love the way the combat system unfolds. Batman: Arkham City unfolds in a similar fashion, with moves being added over time, but you exploring the same areas and the combat changing over time as you explore the combat in those areas and it changes the gameplay over the course of the game. The animations and the way they made it look when you fight a large group of people is incredibly fun for me.
Secondly there’s the climbing and exploration. I just love the way they did the dynamic system in the game where, if it’s jutting out of a wall and it looks like you can grab it, then your character will grab it, or use it to leverage himself higher. It all looks smooth and just so intuitive. Every other game always seems to have a more fake-looking fundamental system in play where they script some things to be climbable and other things not to be climbable.
Third, I loved the way they used the crowds of civilians and how your character interacts with them: sliding past them, or pushing them out of the way depending on whether you’re traveling high profile or low profile. I also love how you can blend in with roaming priests and get lost in the crowd.
Finally, I love, love, love the framing of the game. Having you explore “genetic memories” explains away so many conventions of games in such an elegant way that it just tickles me and brings a smile to my face just thinking about it. Walls of shimmering code block access to parts of the city that you don’t have the genetic memories for yet, health bar is replaced by a synchronization bar that measures how much you’re following real genetic memories, the way crowds of people are meaningless crowds of people in a city that you don’t interact with because it makes sense that the character doesn’t have strong genetic memories of interacting with nameless faces in a crowd. And so on. It all just fits so well in a video game and video gaming open world conventions.
Plus there’s the superficial reason that Assassin’s Creed is gorgeous. Unlike the sequel, the lighting is always pitch perfect. It’s filtered with a certain type of lighting like a stylized hollywood movie with a lens filter and special effects that make everything look cool and gorgeous. (The sequels instead replaced this with a lighting model based on a day/night cycle instead).
The biggest factor is the combat, though. If you don’t enjoy the combat and blending in with the crowd, or running away from guards across rooftops, then the very things you’ll be doing moment-to-moment in the game aren’t enjoyable. And then, why would you play the game? If you don’t enjoy those few fundamental things, then I can see why it would be a game where “they forgot to include a game in it”. I would probably feel the same way about Batman: Arkham City if I didn’t enjoy the combat model in that game.