Connor will spend much of his time in and around his mentor's Homestead, a sprawling estate with acres of land. The simulation even has animals and weather now. You can spend hours hunting in the snow, hiring new artisans to hang out at the Homestead and provide raw materials for crafting better gear, and sending out trade convoys by land and sea. Or you can not.
I've heard complaints that Assassin's Creed games are supposed to be about Assassins, and that all this fluff dilutes the experience. I don't agree, but I see the point. But unlike in the previous games, where world-building missions were a constant distraction, AC 3 rarely insists that you spend time on non-essentials. The RPG elements take a back seat for once—I ended with the same ammo counts as when I started, and was none the worse for it.
Likewise with the naval battle missions; most are optional side quests. For the record, though, they're awesome. Taking control of a massive ship, with its weight and crew and slow-reloading cannons, couldn't be more different from the core AC gameplay, but I devoured the naval missions every chance I got. They're gorgeous and cinematic, and infinitely more interesting than any of the series' previous distractions.
There are no drastic changes to multiplayer, though a new mode called "Wolf Pack" provides more cooperative fun than past games' team-based game types have. It's another distraction, though an intriguing one for sure.