There are absolutely irreparable consequences to your actions in this game. You have to "plan ahead" in the sense that you have to be sure what path you want to go down because other paths will become closed or non-existent. It also is sometimes not obvious which path makes the most sense to take, which is by design.

Without trying to spoil anything, I made a mistake with one of my characters which caused them to permanently leave the group and I can't get them back.

Keep in mind, while this is a video game, it's based off Dungeons & Dragons 5e. Many choices have lasting effects.

tl;dr Don't approach this game as a traditional action RPG game. Treat it like it's the table top game you'd play with people with one person (the DM) weaving a story for you based on what you do in your adventure. Or like a choose your own adventure book and you didn't take your figure off the page you came from...

Far as I can tell, there's no respawns. You do not grind gear/experience. If you avoid doing stuff you are by nature, less experienced then someone who does more. Quests can be resolved because of actions you take that don't directly relate to the quest. Example (without being a spoiler), you are tasked with rescuing someone... but decide you're going to pack it in for the night instead of addressing the problem right away and that person dies. The quest is resolved. Not all quests must be resolved one way, many have multiple options and outcomes based on what you choose to do or not do.

You can also lose access to certain vendors. But you can also gain access to better vendors. (Or you could rob the vendor, or murder them and take everything). But at the same time, you don't need to be as focused on gear than you would other games. Gear is more about additional options. The shields you find at the very beginning of the game, have the exact same AC as those at the end of Act 1 (+2). Wouldn't surprise me if the 'best' shield only has +4 AC. But the shield might have active or passive abilities and that's more so why you'd want it. It's not like say... Diablo were you absolutely wouldn't think of using a starting weapon later. But in DnD, the damage you do is based on what dice gets rolled for it (say a d8 or a d10) + your modifier + any enhancements it might have. But an end game swords base damage (excluding say, special effects) would hit just as hard as a starting sword. But maybe your build lets you swing that sword more than once... maybe you took a warrior/warlock build and someone hasted you...

It's a game where choice matters. Dialog options matter (some in the short term, some long term). Go up to a random drow and tell her "fuck you and the horse you road in on" don't be shocked a fight breaks out. Tell a devout follower that their god is weak and stupid, they might attack you. Try to fake drinking something only to fail the dice roll and you might get jumped by a large number of enemies who no longer trust you unless you can talk your way out of it.

Hell, even doing events in different orders can change events and options.

So you're not going to see/get everything on your first play through. And that's fine because it's going to spark your imagination for next play through. Example, I've seen a lot of gear that empowers you with lightning... and gear that makes me immune to being electrocuted, and boots that electrify the ground if the ground is wet. Right now I'm a smiting paladin, but maybe my next playthrough I play a monk who's just got an electric personality...

Here's a place where modders are your friends. There's a mod for all items already out there that just dumps a bag in you camp with every item in the game. Not something I'd do at the start of the game, but as you play, if you realize you missed something, just mod it in.

For example, I just found out that I made a.choice at the end of Act 2 that skipped out on a strong weapon. It's tied to a specific companion's personal quest, you have to bring them, and you have to select a specific dialogue choice essentially asking for a reward. I'm playing as a pretty run of the mill good guy, so I figured "hey, no need to reward me, just doing good for the sake of it." That was like 40 hours ago. I'd like to play around with that weapon, so I'm gonna mod it in.

Otherwise, a lot of the strongest gear is available in shops or along major quests. There are, however, a lot of items hidden behind obscurity. And the game does lock out old areas after a while, but usually prompts you before doing so. It's also probable that some things are story locked; make certain choices, get certain results. I don't think you can get absolutely everything in one playthrough.

I'd say it's a great enough game that the FOMO can take a back seat, but that's just for me.

Missing gear has never been a worry for me. The game floods you with so much loot no matter what you do.

There is serious FOMO with story decisions, though. There are certain paths that can cause companions to permanently die or leave the party, cause entire factions to aggro on you at once, or kill off important NPCs. Some of these decisions also hinge on die rolls. You have to be prepared to let it go or embrace the save scum.

You can't always come back. I would say most things give you 2-3 chances and the order doesn't matter but certain decisions are a one time chance only and if you miss it, it's gone. And if you miss those 2-3 chances, things are also gone for good.

For weapons the best example is during the "Prologue". While you are still on the Nautiloid there is a strong enemy with a burning sword. If you play as "intended" and ignore the enemy, you won't get it. And there is no option of getting it later (to my knowledge).

Then there are a lot of other decision where you aren't sure of any long-term consequences. During the first 3-4 hours this bothered me and I did a lot of reloading trying to find the "optimal" way. But by now I embrace it. I finally have a solid idea behind how my character would act and just go with decisions inline with that. And I will have multiple play throughs of the game almost certainly.

If you’re confused about what this game is like, look up some lets plays of Divinity Original Sin or Divinity Original Sin: II. Stylistically the game are of a kind, only difference is the Divinity series is using its own RPG rules to govern the combat and dialog options where BG3 is using an adaptation of DnD 5e game rules. The reason i say check out the intellectual predecessors to this game is to not spoil any actual content in BG3.

If you watch those games be played and they seem fun to you, you’ll love BG3, if it seems foreign and confusing and not fun, then don’t play BG3–or pick up DoS and DoS:II on sale if you want to wet your beak on the type of games these are.

I am still in Act one.
But it seems I screwed up some things which let to one character leaving my party early and a second refusing to even join it.
A third character got killed in a scripted event. So I am now stuck with the same three companions for the rest of the game.

While I don't like how this turned out I decided I won't reload just to try to make it better.
I just sticked with my group and Play on because it felt right to do so.

Of course I will miss out dialogues and story elements of the other characters, but I still can concentrate on them in a second playthrough.

I missed out on a great robe for my warlock by accidentally progressing Act 2 before completing quests. However it’s no big deal as there is a bunch of gear in the game and the combat isn’t exactly challenging anyway.

I killed scratch.