Verisimilitude in a campaign world helps the players immerse themselves into the game. If the DM can make a world that feels real, it stops being a simple tabletop game. It becomes a place where the players can leave their mundane lives behind for awhile, and experience the wonders of another world.
That, my friends, is the magic of the pencil & paper RPG.
On the other hand, a badly designed game world makes it harder for a person to suspend their disbelief.
This is one of the major reasons I dislike the 3rd and 4th editions of D&D. The characters have undergone massive power-creep, everybody is playing some exotic demi-human race (drow, tiefling, warforged, dragonborn, etc), and everybody is wearing two inch thick spiky armor and slinging around oversized, overstyled weaponry that is so tacky and tasteless it would make Gil Hibben blush with shame.
Nowadays elves teleport naturally, and unwanted magic items can be ground down into magic dust and traded at the local corner magic shoppe for something better. A character that's been pin-cushioned with arrows can take a 6 hour nap and wake up in full health.
4th ed D&D feels so gamey and fake that it's hard to take it seriously. I realize that it was purposely engineered to emulate MMOs (WoW in particular), but the mechanics and aesthetics that work successfully in a video game do not translate well to a traditional pencil and paper RPG.
It also doesn't help that the game's focus is on tactical boardgame-style miniatures combat. In the old days, we didn't even use miniatures (except occasionally for party order). All of the combat was completely imaginary, and that's a helluva lot more immersive than moving pieces around on a board.
This is not to say that I'm opposed to the use of miniatures. But I do feel that they should be an occasional aid to combat rather than the focus of it.
But I digress...
As much fun as it is to poke fun at 4e, that's not why I'm here. But I do believe that 4e offers some insight into where things went wrong. As I mentioned above, the 3rd and 4th editions of the game have gotten much more outlandish, and this has adversely affected the verisimilitude of the game.
One of the cornerstones of my game design philosophy is that fantastical elements should be used sparingly. I try not to use a fantastical element when I can achieve the same thing with a mundane one.
Take the giant spider for example:
Giant spiders are so common that players never bat an eye when they're encountered, even if the player is arachnophobic. The only worry they have is whether or not they'll be poisoned. The player understand that giant spiders are just there to provide XPs and treasure, and as a result, they don't have any real fear of giant spiders.
So what I've done is to make small spiders scary. Take the following example:
Skeef the thief: Okay I'm searching behind the bookcase...
DM: (rolls dice) You feel something scuttle across your hand, and then a sharp sting. Roll for poison.
Skeef: Sonofabitch! (rolls dice) Whew, saved! What bit me?
DM: You see a small dark spider scurry into the crack between the bookcase and the wall.
Skeef: Little bastard! So did I find anything in the bookcase? Secret door, hidden panel, gemstone?
DM: No, but that bite is really starting to hurt. You can actually feel it throb every time your heart beats.
Skeef: What the hell! I saved, didn't I?
DM: (Shrugs noncomittally)
Eric the Cleric: Let me see that. Does it look like any kind of bite that my character would be familiar with? I have healing as a skill.
DM: It looks like a bug bite. They all kind of look the same.
Blighter the Fighter: Hey, if you two are done making kissy face, can we take it to the next room?
Skeef: Okay, am I like, losing any hit points or anything like that?
DM: No. It just hurts like hell. But you probably won't be picking any locks or pockets any time soon.
Blighter: Alright we go into the next room. What do we see?
DM: You see all sorts of torture equipment; a rack, an iron maiden, branding irons... and a couple of rotting human corpses. You see them pick themselves off the floor and they start walking toward you...
DM: Alright, guys, you've sent those corpses back to whatever hell they came from.
Bligher: Hell yes! What do we get?
DM: Well the corpses are just wearing rags, so there's nothing worthwhile on them, but you can feel free to search the room. Oh, and Skeef? Your hand doesn't hurt so much anymore but you're starting to feel really nauseated.
Skeef: Whaaat? I though I saved!
Eric: Is he gonna be alright?
DM: (Shrugs and pretends to look innocent)
Blighter: Alright, there's nothing we can really do about it right now anyway, so lets get this place searched.
DM: (after a few minutes of poking around): You find an old bundle of rags with an ornate dagger and a small bag of mixed coins. There's also a bunch of torture equipment, but the only thing that particularly stands out is a pear of anguish with some gold inlays.
Skeef: Can I estimate it's value?
DM: Actually you're feeling pretty fucked right now. The hand that was bitten is numb, and you feel like your almost completely drained of strength. Right now the only thing you want to do is curl up in a corner and go to sleep.
Skeef: Wait... am I gonna die?
Eric: I though he made his poison save.
DM: Posion is a funny thing. Sometimes there are modifiers... and sometimes even if you save, you're still kinda messed up.
Blighter: So we have no idea if he's going to be okay?
Skeef: Can we please find an antidote or something? Cause this sucks.
Eric: Why don't we go back to town and get some help? My church will cure him for a small donation.
Skeef: It took us three days to get here, remember? I might die by then, or lose a hand or something!
Blighter: Is there anyplace else we can go that's closer?
DM: The village of Milkweed is close by, but it's tiny and doesn't have a church of its own, just a shrine to Hengus. Brother Lemple, the guy who runs it is even less of a priest than Eric. However, there is supposed to be a woods witch that lives in a cave on Red Bear Ridge. It's a few miles from here. You could probably find it in a few hours.
Eric: I really don't like the sound of that. Witches are bad news.
Skeef: Hey, I'm the one that's dying here. I vote for the wicked witch!
After an episode like this, your players will be paranoid of even small spiders. Not every bug bite needs to be this bad (and it will become predictable if you do this too often), but the occasional serious bite will keep your players on their toes and give them a healthy respect for bugs.
When it's time to take things up a notch, you don't need to go all the way up to a 10 foot tall spider. Let's imagine for a moment that Skeef is crawling though a small tunnel with nothing but a candle to light his way. He comes to a bunch of spider webs and this time he pokes the candle at the webs to burn anything that might be hiding in them. He's also wearing leather gloves to protect his hands.
He's making good headway, until he sees something in the darkness. It's a spider... a BIG spider. The body is the size of a rat, and the legs would spill over the edges of a dinner plate. It pivots on the spot and turns to look at Skeef. And then it jumps on his face... and his candle goes out.
Now that's a memorable encounter!
It's a hell of a lot more interesting that simply using giant spiders as dungeon fodder. By establishing that tiny spiders are lethal, I have made it possible for the players to relate to their characters in a way that would not be possible if I just threw a giant spider at them.
Something that big is too hard for the player to grasp and it pushes them out of their suspension of disbelief. They know that giant spiders don't exist, and so it's safe to drop their emotional guard. That giant spider could just as easily be a venomous Klumvark (a creature I just made up). The creepiness of the giant spider is lost because it's just a normal monster.
The second spider in the tunnel is big, but not so big as to be unbelievable. There are real-world spiders about that size, so the player cannot take refuge in the knowledge that this is a purely fantastic creature. Thus, he is still subject to the natural human fear of spiders.
This is exacerbated by the fact Skeef is in a tight tunnel, and unable to defend himself properly. If he was on his feet he might be able to kick it, or stomp it, but on his belly, in the dark he's at a serious disadvantage. All of this is something that Skeef's player can personally understand. And because it is plausible, there's an emotional edge to his fear that would not otherwise be there.
So let us assume at this point that the party is sufficiently bug-o-phobic. But they've managed to do pretty well for themselves, and the DM hasn't thrown anything truly scary at them for awhile, and they've gotten a little complacent.
Unfortunately, they keep running into these guys with webs tattooed on their bodies. These guys are apparently part of a spider cult, trying to summon their horrifying god to this world. Eventually the PCs find their temple hideout and lead an expedition to clean it out.
The cultists themselves are easily dealt with and the PCs but when they enter the inner sanctum they find that it is full of spider webs. And when they look up, there's a 20 foot spider clinging to the ceiling. Apparently the PCs were too late to stop the summoning, and the god-spider is now climbing down the wall to close off the exits...
Cue epic boss battle.
Now it's important note that the bigger spiders are treated as special encounters. This is not something I would use on a regular basis, or else it would lose its impact. The only spiders commonly encountered are small ones.
This goes back to what I said in the 1st article: When everything is special, nothing is special. By treating these as unique encounters it gives them much more emotional punch than they would otherwise have.
The larger spiders are scary because they are unexpected and because there has already been some build-up to the encounter to raise the emotional tension. But you lose that inherent creepiness of spiders if you use them too often (this is true of any creature that you want to use to cause fear in the players).
By using less special creatures overall, and by making the encounters that do feature them more memorable, you make the game feel more dramatic and believable.