I figure since this is the only major story expansion, it deserves its own thread. Some general info from Bethesda.net:
A new case from Valentine’s Detective Agency leads you on a search for a young woman and a secret colony of synths. Travel off the coast of Maine to the mysterious island of Far Harbor, where higher levels of radiation have created a more feral world. Navigate through the growing conflict between the synths, the Children of Atom, and the local townspeople. Will you work towards bringing peace to Far Harbor, and at what cost? Far Harbor features the largest landmass for an add-on that we’ve ever created, filled with new faction quests, settlements, lethal creatures and dungeons. Become more powerful with new, higher-level armor and weapons. The choices are all yours.
Then there's the $25 price tag - all of the story expansions for Fallout 3 and New Vegas were $10-$15, right? Far Harbor is $5 more than what Dragonborn cost for Skyrim, and $5 less than what Shivering Isles cost for Oblivion... I think. At any rate, the price tag and the promise of the "largest landmass we've ever created" gives me the feeling this is going to be a damn proper expansion. They didn't mention any workshop settlements in their description, though - hopefully there still are places on the Far Shore we can build.
And finally, my pet theory: This might be a "sequel" of sorts to Point Lookout. Desmond implied that he and Calvert were only a part of a larger "great game", and his next target was somewhere up north - if you didn't kill him. And the introduction ("go on a search for a young woman") sounds an awful lot like how Point Lookout started. Nadine might not be that young woman, but it would be nice to have her reappear adventuring on the Duchess Gambit. She'd even make a cool companion.