I'll start with a few things I dont like:
1. 4-choices dialogue system. Its not as bad as I expected but in many cases it feels forced and limiting. Dont get me wrong, the actual dialogue writing is good, much better and more natural than Fallout 3 ever had. But the system has some problems. Sometimes its hard to predict what your character will say when you select a line (actually there is a mod that shows full lines before you select them but I played w/o it). And there isnt enough room for stats/perks checks other than speech/charisma.
I think they did this primarily to limit the ammount of lines they had to record, since in this game the main character is fully voiced. Anyway, before release Bethesda said that Fallout 4 has more lines than Fallout 3 and Skyrim combined. And its not a lie.
2. Radiant quests. Those semi-random, repetitive and usually infinite tasks. Im OK with their existence but they should have only unlocked when there is nothing else left to do. Or at least moved to a separate quest category. But instead developers decided to mix real hand-crafted quests and random stuff together. If you arent aware of this you can be easily overwhelmed with those infinite tasks and come to a conclusion that most quests in this game suck, which is not the case. In fact most hand-made quests are very good.
3. Most towns are locked away or hidden in the beginning. Usually Fallout games have a lot of towns, and player always had a good starting point. In Fallout 3 you immediatelly had access to Megaton, a decent-sized town with interesting characters and quests. Fallout 4's "Megaton" is your old pre-war hometown and its completely empty in the beginning, little more than a playground and settlement-building tutorial.

And now to what I like:
1. The world. Its big, its great, and its very immersive. There are a lot of places you can go to and all of them are hand-made. There you wont find a ton of similar caves TES games are notorious for. In Fallout each location is unique and has some kind of backstory. The environments themself are very immersive, some places you wont forget for a long time. And ofc its Fallout so the world is a little (well, sometimes not so little) crazy, in a good way.
2. RPG. I really like how Bethesda approached this aspect of the game even if a lot of people would disagree. Fallout 3 was full of pure evil choices, and while I think that having a lot of choices is good most of those choices had no sense at all. Maybe if you roleplay totally mad psycho they would. But often there was no actual motivation behind those actions. This is also true for some choices in older Fallout games, in fact they often had harsh penalties for doing something the "wrong" way and possible rewards were rarely tempting enough.
In Fallout 4 most choices are morally grey, rarely there is a clear good or evil side, it resembles a psychological test. What you pick usually depends on your beliefs and information you managed to gather in your adventures. Idiologies and beliefs play a major role in this game. And unlike games before it Fallout 4 never tries to preach moral (characters and factions themself sometimes do, but they usually contradict each other) and very rarely gives clear answers or solid evidence supporting a single view. It simply makes you do a choice based on your own understanding. This is also why I think there is no usual consequences slide show at the end of the main quest. You make your choices because you believe it will lead to something, having developers telling what it "actually" leads to would destroy this.
Companions play much bigger role now, all of them have their likes and dislikes, and occasionally participate in your conversations with other characters. They also often react to the world giving comments about locations or interacting with other characters on their own.
3. Combat and crafting. Aside from doing quests and interacting with different characters the main flow of the game is "explore > kill > loot > craft > explore harder areas". All steps are important and in my opinion all of them are very well made. The exploration part was covered before, this world is worth exploring on its own. Then the combat. Basically its a shooter, 1st or 3rd person depending on your preference, kinda old school shooter with no cover system or health regeneration, which is a good thing in my opinion. The gunplay is solid and doesnt feel clunky like it did in Fallout 3. Ofc there are a lot of different weapon types, and a nice stealth mechanic to provide more options. VATS (semi-turnbased auto targeting system) is way more usefull thanks to a lot higher actions point regeneration rate.
Once you kill stuff you grab the loot and improve your equipment with it. The crafting system gives a lot of options, its not just for damage/defense improvements, you can install a lot of different mods changing items to complement your combat playstyle. Another good thing - settlements building and development is completely optional, you can totally ignore this part and find all stuff you need in the game world.
4. Difficulty and progression. At last, Bethesda's game thats not a total walk in the park. Even on normal first few hours can be challenging, harder difficulties, especially revamped survival, make it brutal. Yes, you get power armor early in this game, so you can feel godlike for a few minutes. But in fact its an old rusty model in very poor condition and only one half-charged power core, so dont expect it to be usable for long. Also most enemies in this game dont automatically level up with you, enemy levels are tied to their zones making leveling up and improving your gear very important and rewarding.

Overall, I highly recommend this game regardless if you played or not previous games.

I'll start with a few things I dont like: 1. 4-choices dialogue system. Its not as bad as I expected but in many cases it feels forced and limiting. Dont get me wrong, the actual dialogue writing is good, much better and more natural than Fallout 3 ever had. But the system has some problems. Sometimes its hard to predict what your character will say when you select a line (actually there is a mod that shows full lines before you select them but I played w/o it). And there isnt enough room for stats/perks checks other than speech/charisma. I think they did this primarily to limit the ammount of lines they had to record, since in this game the main character is fully voiced. Anyway, before release Bethesda said that Fallout 4 has more lines than Fallout 3 and Skyrim combined. And its not a lie. 2. Radiant quests. Those semi-random, repetitive and usually infinite tasks. Im OK with their existence but they should have only unlocked when there is nothing else left to do. Or at least moved to a separate quest category. But instead developers decided to mix real hand-crafted quests and random stuff together. If you arent aware of this you can be easily overwhelmed with those infinite tasks and come to a conclusion that most quests in this game suck, which is not the case. In fact most hand-made quests are very good. 3. Most towns are locked away or hidden in the beginning. Usually Fallout games have a lot of towns, and player always had a good starting point. In Fallout 3 you immediatelly had access to Megaton, a decent-sized town with interesting characters and quests. Fallout 4's "Megaton" is your old pre-war hometown and its completely empty in the beginning, little more than a playground and settlement-building tutorial. And now to what I like: 1. The world. Its big, its great, and its very immersive. There are a lot of places you can go to and all of them are hand-made. There you wont find a ton of similar caves TES games are notorious for. In Fallout each location is unique and has some kind of backstory. The environments themself are very immersive, some places you wont forget for a long time. And ofc its Fallout so the world is a little (well, sometimes not so little) crazy, in a good way. 2. RPG. I really like how Bethesda approached this aspect of the game even if a lot of people would disagree. Fallout 3 was full of pure evil choices, and while I think that having a lot of choices is good most of those choices had no sense at all. Maybe if you roleplay totally mad psycho they would. But often there was no actual motivation behind those actions. This is also true for some choices in older Fallout games, in fact they often had harsh penalties for doing something the "wrong" way and possible rewards were rarely tempting enough. In Fallout 4 most choices are morally grey, rarely there is a clear good or evil side, it resembles a psychological test. What you pick usually depends on your beliefs and information you managed to gather in your adventures. Idiologies and beliefs play a major role in this game. And unlike games before it Fallout 4 never tries to preach moral (characters and factions themself sometimes do, but they usually contradict each other) and very rarely gives clear answers or solid evidence supporting a single view. It simply makes you do a choice based on your own understanding. This is also why I think there is no usual consequences slide show at the end of the main quest. You make your choices because you believe it will lead to something, having developers telling what it "actually" leads to would destroy this. Companions play much bigger role now, all of them have their likes and dislikes, and occasionally participate in your conversations with other characters. They also often react to the world giving comments about locations or interacting with other characters on their own. 3. Combat and crafting. Aside from doing quests and interacting with different characters the main flow of the game is "explore > kill > loot > craft > explore harder areas". All steps are important and in my opinion all of them are very well made. The exploration part was covered before, this world is worth exploring on its own. Then the combat. Basically its a shooter, 1st or 3rd person depending on your preference, kinda old school shooter with no cover system or health regeneration, which is a good thing in my opinion. The gunplay is solid and doesnt feel clunky like it did in Fallout 3. Ofc there are a lot of different weapon types, and a nice stealth mechanic to provide more options. VATS (semi-turnbased auto targeting system) is way more usefull thanks to a lot higher actions point regeneration rate. Once you kill stuff you grab the loot and improve your equipment with it. The crafting system gives a lot of options, its not just for damage/defense improvements, you can install a lot of different mods changing items to complement your combat playstyle. Another good thing - settlements building and development is completely optional, you can totally ignore this part and find all stuff you need in the game world. 4. Difficulty and progression. At last, Bethesda's game thats not a total walk in the park. Even on normal first few hours can be challenging, harder difficulties, especially revamped survival, make it brutal. Yes, you get power armor early in this game, so you can feel godlike for a few minutes. But in fact its an old rusty model in very poor condition and only one half-charged power core, so dont expect it to be usable for long. Also most enemies in this game dont automatically level up with you, enemy levels are tied to their zones making leveling up and improving your gear very important and rewarding. Overall, I highly recommend this game regardless if you played or not previous games.