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.
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.
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.
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.