I've squeezed in about 5 hours of playtime so far.
The good; graphics are decent, with great weather effects (albeit the fog is a bit over the top). Music is overall very good. The gunplay is an improvement over FO3/New Vegas (not very hard, but still). Power armor is very fun to use. Crafting and modding system looks to have quite a bit of depth and thought put into it. The BoS don't seem quite like white knights of the wasteland anymore. The armor system is sweet, I can reinforce my Vault suit with enough cobbled together parts to give it some very respectable DR and still rock the Vault look. I love dogs and I love Dogmeat. Character creator is fairly extensive and gies you a body adjuster, which I like.
The mediocre: Magic equipment returns. I got a leather armband that gives me +1 PER and AGI off a Legendary Radroach. Um, OK. Is this World of Fallout or something?
Writing is usual Bethesda blandness. It's only not ugly because I kinda like Cogsworth and Courtenay Taylor does a good job as the voice actress of my female character, especially considering the script she was provided. After the first half hour the main character seems remarkably unfazed that their spouse was shot, their son was kidnapped, and they are thrust into a 210 years old post-apocalyptic hellhole when yesterday (for them) they were living a nigh on perfect life. At least you got a dialog line when you talk about it with an NPC but it seems the MC's background is not used well thus far. Conversations are boring and uninspired for the most part too.
I am torn about the new perk system. I dislike the removal of skills (and some perks are just skills in disguise, like those that give 20/40/60/80/100% dmg increase to X weapons) but at least some perks seem fun and the higher ranks sometimes add new functionalities. So it's not as bland as Skyrim's perk system at least. traits would have been welcome though.
The ugly: the dialog system sucks. Even when it was first implemented, by Bioware in ME1, it was better. Now Witcher 3, Deus Ex HR and Inquisition have showed us how to make a good paraphrase system, while Fallout 4 has a ridiculously bad one that barely informs you of what you're going to say at all. The ''sarcastic'' option sometimes end up with an harmless quip, but one time I called a BoS Paladin a looter to his face, for which I should have gotten a facefull of laser rifle. I had no intention of saying that.
The quests are bare-bones. At least FO3 tried to give you choices, even when they were silly (IE Metagon). FO4 cuts the middleman and simply asks you to go there and kill those bad things. One quest asked me to go to the Corvega power plant, and I must have killed at least 40 raiders to clean the place up completely. At some point I was beginning to be really sick and tired of gunning down the same mindless raider idiot again and again (the AI being of course abysmal).
Tied into the above, world design is usual Bethesda theme park. For starters, early settlements have 2-3 people, sometimes 5. Raider camps universally have double that, and as I said that power facility alone was home to more raiders than the number of friendly NPCs I had met thus far. Pre-war prologue included. After New Vegas took pains to craft a believable world, and even Skyrim saw improvements in this regard, this is quite a shame.
Performance is bad. My PC isn't the best, but I can run Witcher 3 at over 30 FPS in all but the most extreme conditions, on medium-high settings. Fallout 4 doesn't look anywhere near as good and I get lower FPS on it consistently; urban areas see a pretty big dip in framerate in particular.
Overall, not overly impressed. It's a fun run-and-gun simulator with some promising aspects, but as an RPG and Fallout game it leaves much to be desired thus far. Not a terrible game by any means, but it will have to improve to beat Skyrim in my books, and beating New Vegas already seems right out.