The points at which the game transition between acts seem a bit arbitrary (mainly for Act I to Act II), and I don't see a narrative or mechanical reason to lock us out of previous maps and quests. As far as I remember, previous Baldur's Gate games didn't have this kind of points of no return. Why do you think they did it? Do you like it?
I think the problem is you're viewing it like you would say Diablo. Where those have acts and you can just freely roam through the whole thing.
This is d&d. So this is more of a linear story. More precise your linear story. If your story involves going from the beach to baldur's gate and you never saw the underdark then that's perfectly fine. There's really no right or wrong way to play.
And that's where the massive amount of replayability comes. Because long as you don't keep choosing the exact same things every time you will get a different story.
The problem with this is the encounters and enemies have specific levels. If you skip things (and there are several instances where progressing one quest ends or eliminates many others even when they are not mutually exclusive, a la evil vs. good companions) you could end up very underleveled later on. I had several fights early in Act 2 where I felt I was VERY obviously missing content as I needed my entire party to be a level or two higher to get through without save-scumming, spamming items, exploiting mechanics, or all of the above.
A lot are super not obvious too - in Act 1 there are quite a few things you're "supposed" to do before you go to the goblin camp or else you'll be pretty unprepared to deal with the sheer number of level 3~4 mobs. But I assumed initially it worked like more traditional RPGs and you do the side quests first and the main quest as the last thing before wrapping up the act. You can also sneak around and pretty much get instantly TPK'd by the Spectator until you have the action economy to deal with it (level 5ish). But in Act 2 you're actually "supposed" to go to Moonrise first before going to the Mausoleum or else you lose the ability to finish tons of quests and access to several vendors. You're told to find tieflings, Mol, and Zevlor all at the same time but you have no way of knowing that if you go to the mausoleum first you'll lose access to the tiefling quest resolution, but not the other two. It's not obvious at all that you can't find Mol until Act 3. It's missing the kind of corrections and hints a DM could do on the fly to keep things moving, or at least nudge you in the right direction if you're willing to be railroaded in order to see the most content possible.
Iirc you don't lose access to anything in the mausoleum until you take a dip in the pool, where they quite literally show a popup that uncompleted quests may be broken. If by that point you check the map and the quest log and you have stuff you clearly should do before continuing, like exploring the tower, or freeing th eprisoners they will obviously murder if an assault happens to avoid having to fight them in the case we liberate them in the assault, well, yeah, idk man.
My only gripe is the Zevlor quest and the Mol quests, where the don't really say that you need to do it to see them. Well technically you can go down below through a hole and featherfall or through some stairs after liberating the prisoners, and rescue Zevlor before the pool, but most do it after it. SO my only gripe is Mol, because the journal gives no indication that spoilers of act 2:
spoiler
she was not at the towers
But besides that it's quite clear that once you reach the gauntlet that maybe that's a bigger thing than a simple side quest, specially when the popup shows up.
they quite literally show a popup that uncompleted quests may be broken
It is unclear which quests will be broken, or how many. 15 of the 20 or so quests I had at that point couldn't actually be progressed then anyway, but 5 could.
you clearly should do before continuing, like exploring the tower
How is this clear? I was assuming once I got to Moonrise it would be one long fight to Ketheric and so was trying to do everything else first, imagining it would effectively conclude the act. I had no reason to assume it would be a social zone that I needed to visit and complete quests in before doing the temple. I had no idea there would be an assault on the tower.
it’s quite clear that once you reach the gauntlet that maybe that’s a bigger thing than a simple side quest, specially when the popup shows up.
The pop-up is, but a similar popup happens when you leave Act 1, but many things remain accessible. So it is inconsistent. I think it would help to have quests show recommended levels so it is more obvious what order you're expected to do them in - you could still go out of order like you can now, but I think it would be a better experience.
How is this clear? I was assuming once I got to Moonrise it would be one long fight to Ketheric and so was trying to do everything else first
Well, all Jaheera and Isobel told you to go to the towers to investigate how to disable Ketheric's invulnerability and that you could pass through as a True Soul thanks to your condition, so... yeah? You had reasons to assume that it would be a social zone.
The act 1 to 2 popup doesn't say that previous zones will become unnaccesible and this does. That's a pretty big indicator to me.
By the way, I think that it's possible to go down below through the prison and reach the final zone before dealing with the nighsong, I have to try it in my yet next playthrough.
The level indicator thing would be nice, but this won't happen with Larian, since they don't treat quests like quests you are expected to do, but as "stories" that you write up in your journal. The issue here is that effectively they are quests and that you are expected to do them since they give EXP and you need it not to get stomped anyway. Here I agree with you but don't expect changes.
The game warns you before you enter the mausoleum. There is even a character who warns you. That's exactly how a dm would do. You even have 2 warnings actually. One when you enter the mausoleum, and one when you enter the final zone of it.
I feel like level is a bit less important than people think, at least for BG3.
Lower level simply means less HP and fewer spells to work with. But your damage for the most part doesn't change (except for like lvl 5 as an example). In this video, this guy solos Act 1. It does require more planning and tactics. But if you can solo this stuff, you certainly can do it with fewer levels. Just those additional levels will make it easier.