In short: Really fucking good. The trailer opens with a near-lifelike Blackbeard holding court with his crew and discussing a pirate more fearsome than he, “ranking amongst the most clever” sailing the West Indies. We then see the pirate in question—the game's main character—perform a few feats of badassery, including assassinating an unknown Brit WHILE swiping the poor john's woman. The timeless art of seduction.
The character's name is Edward Kenway. He's the grandfather to whiny Assassin's Creed III protagonist Connor and father to awesome ACIII prologue protagonist Haytham. With Edward, you'll be able to explore 50 different Caribbean locations, hunt vessels to plunder, and gain treasure that can be used to hire a larger crew on your pirate ship. Honestly, this sounds incredibly fun.
One more piece of welcome news: The developers promise this game will move quicker than the series' last release. So a prologue shorter than 10 hours? Beautiful.