As a longtime fan of Bethesda games and with RPGs being my favourite genre, I have to admit I was pretty disappointed with Fallout 4 - and my expectations were pretty measured for the game going in (though it is a great game to be sure).
Myself and 3 of my friends played the game and we discussed it as we played on a Whatsapp group and basically we came to the consensus that it was a great game and worthy of our time and money, but that it'd failed to innovate as much as a sequel of 7 years should, and that it'd also dumbed down several of it's systems in favour of making the game more user friendly.
Those systems referring to the removal of the repair, the removal of the karma system, the changes to the game's leveling system and the changes to the game's quest system. In a quick summary, removing the repair system was disappointing as we'd enjoyed repairing items in past Fallout games as it made the world feel more apocalyptic. The removal of the karma system obviously just removes a layer of complexity to the game and I can see no positive at all in Bethesda doing that. The changes to the leveling system have the negative impact of making it feel like you never really get stronger, as you can go several levels without learning combat perks as the game puts so much emphasis on learning non-combat perks. It also diminished the importance of SPECIAL as they now do less under the current system, whereas they previously added to your skill attributes. And the quest system took on the style of Skyrim, where instead of having 30 deep and complex quests like in past Fallout games, they gave you 300 fairly superficial quests with uninteresting stories and almost zero choices to make. There is a place for both styles, and we'd come to expect that Fallout was the one with 30 and Elder Scrolls was the one with 300 - but now they're both the same.
The game also has a litany of small changes/systems that we didn't like, such as the fact that Power armour is now a vehicle. By making it a vehicle all 4 of us hardly ever used it because we'd have to fast travel back to it if we needed to use it. And you also can't do many things in the game while wearing power armour.
There also weren't many types of regular armour to wear and hardly any new enemy types, and while it's nice to give your character a voice, the things you are able to say are always really generic and throw away - unlike in Bioware games or in the Witcher (which are also voiced dialogue trees). Also almost zero of the new features that appeared in New Vegas and were popular (Faction system, hardcore mode, dynamic events etc.) were in Fallout 4 - I guess because Bethesda didn't work on New Vegas but that's a pretty unsatisfying reason for us to accept.
However.... it was still a great game, albeit disappointing. Most of the strengths of the series are still there.
EDIT: Just reading the OP again. While that user metacritic score is obviously absurdly low, I suspect its a lot of people who feel like Bethesda sold out a little bit with Fallout 4. Skyrim was their most successful game to date and they may have had dollar signs in their eyes when they began production on Fallout 4 because I can't understand why they dumbed down so many of the game's beloved system. Fallout 4 doesn't feel like a game that was made for me. It feels like it was made for people who don't post on Vgchartz and play 1 or 2 games a year and are "curious about this Fallout game everyones talking about". Meanwhile I have not met a person yet who isn't annoyed that the Karma and repair system are gone for eg.