Diamond City makes sense as a settlement. I mean if you set your game in Boston it makes sense that they are going to have to do SOMETHING with the Green Monster. In a lot of disaster films/stories, people are seen gathering in stadiums. The problem for me is that it's 1 of only 2 major settlements in the whole game. Also that it's filled with nonsense like people with European/Russian accents, the stupid radio guy whose only character trait is that he's shy, and a bunch of other stupidity. Also those totally boring, lame quests that you accumulate simply by walking past people, which would be fine if they were actually good quests instead of lazy trash.
What is unacceptable is that exploring the Sariff Industries Headquarters in the opening of Deus Ex: Human Revolution is more interesting than exploring Diamond City, because in every possible way Deus Ex Human Revolution is a better RPG than Fallout 4. The level design in Deus Ex matches the writing, just like the level design in KOTOR matches the writing. Fallout 4's level design is a mess and they don't even consider making it believable.