I've been playing Fallout 4 FROST lately, and wow, it's such a totally different experience than the base game.
Throw everything you know about Fallout 4 because it's an entirely new game in itself. It's set just 5 years after the bombs fell, so it's basically still in the nuclear winter state. The air is still toxic, so you need to wear a gask mask or any other appropriate protection while outside. It's mostly safe when inside a building or underground though. There's also no civilization yet, the towns we know in the base game are not a thing yet. There's no story from the base game, every NPC and quest has been removed. There's only one main quest you received at the beginning, which is to survive. But it doesn't mean that there's no story in FROST, there's a story but it's not handed to you. You have to find it, whether from notes you can find here and there, or from NPC chatters and so on. More or less like the Souls series.
Everyone, including you, is a "survivor" and most of them will either shoot you on sight or at least give you some warning first. There's only a few traders who buy and sell at outrageous rates, because everything is scarce. Ammos are super rare, you'll be forced to use melee weapon lots of times to conserve ammos. The mod encourages you to not get attached to a weapon as you'll run out of ammo soon and forced to use another weapon with different ammo. Some enemies are tweaked, like the ghouls that have super armor on everything but their head. Sounds hardcore, but they're manageable with a melee weapon as you can block their attack and counter with a slash to the head. One swing will usually do the job done. The animals aren't all fully mutated yet. There are still dogs and wolves, as well as cows, mutcrabs and huge rats. Yao Guai is still a bear, but still half mutated, etc. Overall, the combat is mostly deadly. You'll be dead in a shot or two, and it's the same for most enemies too.
There's this new thing that makes it more interesting, the sanity system. You start with 100 sanity, but every time you kill a survivor, eat uncooked food, taking jet, etc, you'll lose 2 sanity. Killing a child will net you -10 in sanity. If you hit 0 or lower sanity, you'll go insane which leads to some pros and cons. There are sanity restoring item (mostly food) so it's best to keep an eye for them if you want to stay sane. There's also some perks that make use of the sanity system, like if you stay 90% or more sane you'll deal 15% more damage. The sanity system also adds a lot to my role-playing experience, it made me think before killing a group of survivor who are just chilling in a small hut. Do I kill them for the loot or should I just let them be? They're just like me, trying to survive in this world.
All these systems are so well integrated so it creates an entirely new gameplay loop and experience. You need to be careful of your surrounding at all times, because getting ganked by a small group will be the end of you. You don't have to kill everything you need, sometimes it's best to avoid the enemies you encounter. Finding a bed and/or a workbench is like finding a bonfire, a huge relief in this unforgiving environment. It's advised to play in survival mode but I find it too overwhelming, so I tinker the rest/hunger/thirst rates with another mod so I don't have to constantly thinking about them. I also use another mod so that I can save in a bed without sleeping instead of having to sleep first to save.
Overall, it's such an immersive and mesmerizing experience.