Out of the box, Fallout 4 is much, much better than Fallout 3 or New Vegas. Both of those games were utterly unplayable for me until heavily modded, whereas Fallout 4 has been extremely stable with workable mechanics. I'm not running a single mod and yet I've happily put in over 250 hours so far. Heavily modded, both Fallout 3 and New Vegas were better to me, but I'm sure that Fallout 4 with its mods will eventually be by far the best experience.
The good:
It's really, really pretty outside. You can see a long way, and only occasionally is pop-in noticeable. Compare that to 3 or NV where the landscape often completely changed as you grew closer, so that you could see the valley from far away but closer up you can't because there's a building in the way.
It's really, really stable. Not just "for a Bethesda game", but for any game its size.
It's really, really shooter-friendly. This time the combat mechanics are honestly well done by Bethesda.
The voice acting actually improved compared to previous Fallout games. Evidently all the voice actors were not required to travel in the same Vega wagon this time, so not everyone sounds the same.
Lynda Carter does an excellent job acting, singing and writing her material. She's not the world's best torch singer, but she's pretty damn good.
The music is way, way, way better than in NV. Maybe not as good as in 3, with the notable exception of Carter's material, but maybe more varied. (Hard to remember because I used the excellent "GNR Expanded" mod.)
Enemy AI is a bit better, though still embarrassingly bad for 2015.
Map design is often excellent. I really like all the partially collapsed overpasses and partially submerged areas.
Bethesda has discovered that nuclear holocaust doesn't cause the air to be green, nor remove damn near every other color besides brown.
Feral ghouls have become more . . . interesting.
Nick Valentine is probably my all-time favorite companion, possibly with the exception of Willow.
Making settlements gives a real sense that the character is making real progress.
What's not:
The constant repetitive quests to locations one just cleaned out grinds home the lesson that no progress is being made.
Quests in general are far too repetitive. Going to a certain location to kill everything there is exactly the same as going to a certain location to kill everything there to rescue a hostage and exactly the same as going to a certain location to kill everything there to recover a bit of tech that wasn't there the last six times you looted that exact chest.
It's extremely difficult (at least on Survival) early on, and extremely easy at high level.
Companion AI is embarrassingly bad even compared to Fallout 3. Can't count the times Nick has run across my line of fire for no particular reason during combat, and while he always turns his back on me if I speak to him, he always tries to get directly in front of me and face me if I'm about to open fire. If he can't get in front of me, then he'll stick his arm through me so I hit that if I fire. Greenpeace should be so motivated.
Bethesda really needs to decide if the nuclear exchange was two days ago or two hundred years ago. The food is (moldy food excepted) still edible and many terminals still powered, yet many cars and houses have largely decayed away.
The settlement boundaries are arbitrary and senseless. In one case I'm building communal shanties in a cramped little area that is literally surrounded by houses and buildings.
The great promise of your character being voiced was the ability to emote, yet someone apparently forgot to warn the male actor (haven't played as the female) and the writers. Watching my wife be brutally murdered and my son stolen should evoke more emotion than watching my car get towed. It does not. That said, I do really like both actors' voices.
Bethesda really needs to revisit its weapon system. Nothing that weighs twenty-eight pounds should ever be termed an assault rifle. Maybe an assault-on-common-sense rifle.
The map is waaay too small, so everything is crammed into the same area. Come into one raider gang's area and the firefight often draws in the supermutants living next door. Literally, next door. It's like virtually every building one can enter is some gang's home base.
Companions have far too few lines for their programmed speech intervals. I've experienced them saying the same vapid thing four times in a row within a couple minutes. Granted, adding voiced lines costs money, but having them chatter the same thing over and over is simply annoying.
There is zero thought given to how all these beings survive. They evidently eat each other while waiting on me to plant stuff.
People should not accept dead bodies in their homes. Especially military units and settlers.
Dialog has been dumbed down to the point that I just select the bottom answer all the time. It's an atmospheric shooter far more than an RPG.
Character design, textures, and animations are still not equal to the best work of modders.
The verdict:
Although I have more negatives than positives, I'm very happy with the game. I pre-ordered just to support Bethesda making another Fallout game. I full expected to wait months to be able to have a satisfactory experience. Instead, I have enjoyed the game exactly as they designed and released it. Fully modded, I think both Fallout 3 and Fallout New Vegas are better games, but that is because I can choose from tens of thousands of mods to build them into exactly the game I prefer. Since Fallout 4 has that same flexibility and support built in, plus is a really good game from day one, obviously it is the superior game.