First, something obvious but i have to say it.
Sense of danger in a cRPG =
Arx Fatalis.
A fog of war that can be slowly unveiled. I want the sense of pioneers heading into uncharted territory.
I'm not sure how you mean it.
I would rather have some (very limited) areas where there is some natural or unnatural fog and you can't see very far unless you the Artifact lamp of true path (which you can only find off the beaten path, otherwise, your game design is garbage) or a rare spell.
If it's about visibility, like exploring a dense forest and not being able to see very far away, i agree, of course, if it's just having a fog of war for the sake of it, i don't find it very interesting.
I'd rather have a double or triple visibility radius, one for the obvious environment, and you can see it from afar, a second one for creatures, in that case, visibility depend on their size and movement (and object hiding them) and a third one for hidden objects and creatures.
Two layers if fine though.
Going by some recent and not that recent discussions, it occurred to me that what people mean by good exploration can vary greatly. So let's discuss - what do you value when exploring and what you don't care about? And what are the games that do it best?
Personally, exploration is most fun to me when it's the main driver of progression. Consider, for example, Grimrock 2: you only have one overarching objective - to escape from the island - there are no main or side quests beyond that. Instead narrative is structured through level design, in a kind of horseshoe loops: you explore freely until you come upon an obstacle that blocks your progress (a puzzle, a trap, a locked door), then you explore sideways to find a solution. Bonus points if the solution can be accomplished in several ways depending on your character build like in Quest for Glory or, more recently, Prey (although the latter two have more of a mixed, both plot- and exploration-driven progression). And you can contrast that with Morrowind or Gothic, which while having open worlds, progress through a linear succession of quests, largely depriving exploration of any importance.
On the other hand, I've recently found out that I don't care for multiple paths through the levels as much as I thought. Though in retrospect it's quite logical: if exploring a level is fun, then you'd want to explore all of it, leaving no stone unturned. And if it's not fun, even figuring out which path to take becomes a chore.
Many games nailed exploration with a different approach, Might & Magic series, the latest Wizardry trilogy (didn't play the earlier titles), Arx Fatalis, etc.
Didn't like KotOR one as much (multiple optional paths in most dungeons) for example.
Arcanum over-world exploration (Like Fallout but with some hidden/inaccessible locations) works fine too while its dungeon design is terrible.