As a Fallout game:
It's even further away from Fallout than Fallout 3 was. This is not set in the Fallout universe by any means at all, it's set in a stupid futuristic Boston where every other person is a synth who apparently looks just like a human. Shoehorned in, we have vaults, Super Mutants and ghouls. I didn't even come across any vaults other than the home vault and one other, and they were both very boring. Super Mutants, didn't find any explanation as to why they're in Boston. Suppose it doesn't matter. 99.8% of ghouls you meet are feral ghouls who are, again, just zombies.
On the gameplay side of things, what do you expect in Fallout? Skills which determine how you interact with the world, quests that allow for multiple solutions based on your skills, things like that. This game has none. Skills are gone, and perks don't replace them at all - almost every perk is focused around dealing damage. You ARE playing this as a fighter, you don't have any choice. I avoided combat wherever possible and sneaked as much as I could and my total kill count is about 300 people and 230192830128730 ghouls/animals. Frequently, quests will not proceed until you have killed everyone in an area. If you get them to surrender with the stupid Charisma perk, the game waits until you execute them before allowing you to progress.
This is so far removed from Fallout that you really wonder why they used the Fallout license for it. Fallout 3 was desperately trying to be Fallout and failing, but this doesn't even try.
Let's talk about the amount of choice you have in the main quest, because you don't have any choice at all in all the sidequests I played.
Basically, you get strung along by the game with no agency whatsoever until you reach the institute. At this point, there are three factions: BoS, Railroad and Institute. Joining one gets you the quest to exterminate both of the others. There is no way around this. I joined the Railroad, although that was because they were closest to where I was standing when I got the choice. There is nothing compelling about any of these fucking factions and no reason at all for you to pick any over the other.
Here's how the extermination missions went:
BoS: You have to steal a vertibird and fly to their big airship to explode it. To get the vertibird, you must kill about 20 BoS soldiers. As in, you actually must, because the ship won't take off until you go back inside and kill anyone you missed the first time. When the ship takes off, you're on the minigun, in what might be the first ever turret segment without any enemies in a videogame. You just sit on the turret for a boring 3 minute flight to the airship.
Aboard the airship, you get a disguise. This works about as well as the disguise mechanic in Red Faction 1. If you haven't played Red Faction or don't remember, what I'm trying to say is that it doesn't fucking work. You plant a few charges on the ship and get back on your turret as the airship blows up. No other way to do this.
Institute: Shoot a bunch of unarmed scientists and the Railroad teleport in out of nowhere and shoot some more with you. You have to shoot a bunch of people on the way to the reactor, on which you plany a nuke or something which then cinematically blows up. No other way to do this.
The game deprives you of choice at every turn. You have no control over anything other than which explosion you get to watch at the end. It's truly, utterly pathetic.
In conclusion, this is about as much of a Fallout game as Candy Crush is. Candy Crush is probably closer to Fallout, actually.
As a Bethesda game:
All that's important in a Bethesda game is a wide open world full of dungeons to explore, loot to find and the ability to do things (roughly) your way. This game has none of these. The world feels small and cramped, and has nothing of interest. There are very few actual full-on dungeons, mostly just buildings with a single room with a few raiders inside. Shit on Fallout 3 and Skyrim all you want, but they used Bethesda's formula effectively and were engaging purely as combat-based dungeon crawlers as a result. Fallout 4 exhibits a complete lack of ability on the part of the developers to design dungeons or a fun world to explore.
In Fallout 3, Skyrim and even Oblivion you could do things the way you wanted. I refer mainly to character builds here. Let's take a very quick rundown of all the kinds of ways you could approach combat/quests/dungeons in those games:
Fallout 3 - Combat, Stealth, Speech (there were plenty of Speech checks). Important to note that stealth was viable in this game, whereas it is absolutely not in Fo4.
Skyrim - Melee, Ranged, All the schools of magic, Stealth, or some combination of all of them.
Here's the builds you can have in Fallout 4:
Shooter person
That's all. Every dungeon is navigated on Todd's terms, not yours, and Todd's terms involve lots of tedious shooting until you reach the end of the dungeon, at which there is typically no reward.
In conclusion, Fo4 lacks whatever it was that made F3/Skyrim so good at sucking you in and wasting your time.
As a standalone game:
Let's get one thing straight - you've heard people say the shooting is improved, maybe even good. It's true, the shooting is improved from Fallout 3. It's bordering on being decent. What it isn't good enough for, though, is the 20 hours of nonstop repetitive combat the game will force you in to. Boredom reigns supreme as room after room of headshots and VATS kills drags on and on into eternity. It's an absolutely dull nightmare.
This game has fuck all going for it. This section is so short because I can't write about anything other than combat because that's the entire game. I could write about the story, I suppose, so here:
The story is boring, badly paced, makes little sense and expects you to care about factions who you have little to no info on and even less reason to give a shit about.