There are my first impressions only as I'm 5 hours into the Beta:
Came into the game expecting the sailing and ship upgrading to be like Black Flag and was disappointed. The sailing and ship battle mechanics are far better in BF in my opinion, as there seems to be less speed control in S&B and the targeting system is easier and less realistic than BF too.
Not much to do off the ship other than speak to tribes people, but I am hoping that there will be more to do on land as the game progresses.
Certainly not the AAAA game we were promised by Ubisoft, not by a long shot.