Advertised as the largest piece of DLC that has ever been developed by Bethesda, Fallout 4's Far Harbor promises to bring you to the titular location in Maine. Needless to say, this means that there will be radically different enemies, weapons, and perhaps most importantly, people, who you will encounter as you make your way through the radioactive fogs and twisted swamps of Far Harbor.
Of course, what better way to remind people that some giant mutated lobster will kill you than to put out a trailer?
Thankfully, said trailer didn't spoil every plot point in the Far Harbor DLC, but it did show show off some rather fascinating things that Bethesda might be trying to accomplish with Fallout 4's largest DLC to date.
It's totally not like Point Lookout at all!
The most obvious difference between the Commonwealth wasteland and Far Harbor is that it simply is a geographically different place. This means that the usual buildings, streets, and baseball stadiums that you fought in will be replaced by small fishing towns, swamps, and forests. Naturally, this would probably make long range weapons less effective, mostly due to the fact that you can't really snipe someone if you can't see them through thick radioactive fog. Conversely, this might also make fighting in Far Harbor one of the most graphically pleasing and or frustrating things in Fallout 4, at least if prior experiences in Fallout 3's Point Lookout DLC is any indicator.

Speaking of Point Lookout, there is a distinct possibility that Far Harbor may borrow some plot points from the last time Bethesda had a DLC that was based on swamps and cults. The beginning of the trailer starts off with someone asking for the safe return of their daughter (which is likely the start of your Far Harbor questline), and being the good samaritan/ruthless mercenary that you are, you decide to get on a boat and look for their daughter (what can possibly go wrong?).
When you get to Far Harbor, chances are that some small settlement is under attack (cue Preston Garvey to the rescue) from the local chapter of Children of Atom cultists who believe that they are spreading the light of their savior by irradiating fog. Unbeknownst to them, a (possibly escaped) Synth is actually manipulating them for its own benefit from the local observatory, once again proving that Synths are not to be trusted.

Oh look, fresh lobsters and fish!
Of course, religious fanatics aren't going to be your only problem in Far Harbor. Different locations means different wildlife, which means different weapons and armor to kill said wildlife. You've got what one can only assume to be mutated lobsters, giant fish, and other kinds of literal swamp monsters.

To help you fight them off, you could use the same things that you were using while you were in the Commonwealth, or you could use a plethora of new weapons and clothing like lever action rifles (which you might recognize from Fallout New Vegas), some kind of shortened SMG/Assault Rifle, and all kinds of nautical and Children of Atom themed armors.
With a relatively odd release date of May 19 (a Thursday), it can only be assumed that such a date was picked due to its relative proximity to E3, which would ensure that Bethesda would be in everyone's minds, which in turn would make any news of more Fallout 4 DLC even more impactful.