Simply, the most detailed game world to have ever been created.
The New Shooting system is great and much needed. Driving, I think is great and more challenging than before.
I will say a few niggles stopped it being a 10 and they were,
Mission structure has recieved little to no innovation, i was expecting more choice in how and when i approached a mission - aka Hitman, rather than a linear scripted path. This contradicts everything about openworld gameplay and i thought Rock* could have implemented some new ideas to approaching a mission.
Maintaining relationships is just not fun. Driving across the entire city in an in game hour is near impossible and you will lose respect of your friend for being late. The activities just arent that fun to do time and time again. And your phone never stops ringing.
Melee combat is clumsy. When you think of how deep The Warriors and Bully system was ( both Rock* games) i really think they missed the boat on this.........such a shame. This was a huge dissapointment for me.
Multiplayer is great - when you connect!
No rewards for treasure hunting....
Lastly pop up and framerate issues can take you out of the immersion, but arent game breaking.
This really is an amazing game, despite the niggles i have, the score is justified by the shear scale and ambition of the game.