The criteria are simple:
1: What's in it?
2: How easy is it to get in or out?
3: How close to civilization is it?
As a result, something like Daemonheim or the Underground Pass wouldn't count: though frightening and only barely comprehensible, they hardly impact the lives of anyone who isn't already an adventurer.
6: Ashdale Caves
Humans dug too deep underneath a thriving settlement and awoke a danger the likes of which they've never seen. I'd call it Lovecraftian, but they're simply ugly monsters rather than entities that break the rules of reality. Still, those ugly monsters would have removed a city from the map without anyone even knowing they were gone until it was too late... if they weren't so weak that even a novice adventurer could cull them.
Somewhat far removed from civilization, Crandor makes the list for one reason: it was an important city that was utterly destroyed within recent memory: thirty years is hardly a long time, except maybe to the young adults among us. Additionally, Elvarg shows the effect that even a weak dragon can have on a city, if the attack comes without warning. It's Ashdale, except successful.
4: Monastery of Ascension
The idea of people being kidnapped and brutally experimented on is fairly common in both dark fantasy and horror, especially considering the secrecy around it, as it's located in an area that's already filled with life dangerous enough to cause unexplained disappearances. Perhaps the only thing keeping it from placing higher on the list is that the area around Yanille is fairly detached from human society, and already equipped to deal with large, hostile things... Of course, being eaten is most likely preferable to having your humanity stolen from you in the name of a cause you weren't even aware of.
3: The Wilderness
An enormous, desolate wasteland filled with all manner of unnatural creatures, that only just reaches the borders of some of the largest cities in the world? Luckily, the areas closest to the border would be relatively safe even if they weren't marked: you'll only be attacked by the truly dangerous creatures if you make the effort to seek them out. The real danger is from other people, and literature is quick to remind us that people are often the real monsters.
2: Fremmenik Slayer Dungeon
Let's look at the facts here: some of the people in Relekka can probably see the entrance from their homes, and it's only a short walk to Seer Village from there. It's not a small entrance, either: far larger than even the biggest monster inside it, without any obstacles of note. More than that, there are a lot of things inside it, and while they're hardly a threat to the average adventurer, even the smallest and weakest creature there is impossible to kill for anyone that isn't a specialist. Luckily for the surrounding settlements, they seem content to segregate themselves into their tiny caves, though I'd prefer to think that the point of the slayer skill is to keep the population at safe levels.
We're trapped! TRAPPED! AAAAAA!