It's an Assassin's creed game. By now, you know what that means. An amazing level of production, a great story, just moving around will be fun, but you will be doing the same kinds of missions over and over while combat is way too easy to master.
They tweaked the parkour a bit so you are less likely to jump in a direction you did not want to or accidently fall. You also will no longer be buying buildings. Instead, you will be recruiting people of different skills to build a village and craft various goods to sell and new equipment. This is where you can make the most money, but it gets tedious with a really clunky menu that only lets you craft one item at a time when you may wish to craft dozens.
You do recruit assassins again, but this is really stripped down from AC2. There are missions to send them on which only earn you small amounts of money and experience. It is actually faster to level them by calling on them to kill people in the field. And you will not be harmed by ignoring this feature all together if you wish to.
The combat has changed some. Flintlock pistols being your primary ranged weapon ranging from single shot models to multibarrel and even duckfoot designs. Though they do take considerable time to reload leaving the weapon useless once empty. You can upgrade to carry two pistols at once making this less of a problem.
The melee combat has a few tweaks that will make it seem difficult at first. But this will change as you learn how to handle each enemy. Killing dozens of enemies without suffering so much as a scratch will become common as you play on.
My only real complaint is the same one I have had with every game in this series. So much busywork. Most of my time was spent collecting items and doing simple delivery missions. These can be skipped, but it will be kind of a short game if you do so.
All in all, a great game with a great story with the few flaws you have come to expect. Also, naval battles!