The game was large which is nice.
Upgrades: your able to upgrade your ship, your guns and swords, and craft an assortment of gadgets and clothes; some clothes even give special effects (which is awesome).Early in the game you obtain an island which is upgrade-able.
The online multiplayer is fun, and different from norm, taking a stealth approach, with multiple game modes. Even a 'build your own game mode', where you can decipher game mode, map, rules, restrictions, and play it with other players.
The treasure maps were a cool idea.
Plus the fact your a flippin pirate, that can sail a fairly large sea.. That was a new and cool to experience.
There where chests throughout the game along with another thing called 'Animus Fragment'. These things in specific got more annoying to collect and felt more like a chore later in the game.
Towards the end it felt pretty null game play, i'd felt like I had done everything i wanted to even though my percent was still at 86%, so I didn't feel it had much replay-ability.
The game was well rounded.
That being said if you love games its worth the time.