As a Dungeon Master, I always try to think about how the adventure, encounter, or situation would look if it were a work of fiction. How would the story read? How would it look on television? How would it sound in a movie? If it would seem absurd or unbelievable in that context, I have to ask: why would I expect my players to buy off on it? This kind of thought process really drives my development of adventure material.
Here is a brief illustration of what I mean:
* * *As the Fellowship descended deeper into the Mines of Moria, they came upon a large chamber. Frodo looked in awe at the cavern but quickly noticed a distinct odor. He drew a deep breath, trying to identify the smell. "Is that ... blood?"
Gimli shook his head. "No. It's rust." He pulled his axe close, shaking his head slowly. "I have heard legends of something like this, but..." From the darkness, a high pitched hiss erupted, interrupting the stout dwarf. He pointed towards the dark corner of the chamber. "There!"
A peculiar, reddish beast stepped out of the shadows. Gandalf stepped forward, holding his staff out defensively. "Be cautious! It is a rust monster! It will consume the very metal off your bodies." The wizened man looked towards Frodo. "Be careful, Frodo. Although the Ring's dark power protects it from the creature's attack, the beast will make quick work of Sting."
Frodo looked to Gandalf, concerned. "But, Gandalf! I thought Sting was a magical blade?"
Gandalf shook his head. "It does not matter. The rust monster destroys magical metals as readily as normal metals. Be on your guard."
Right. Had I been reading that book, I suspect I would have put it down and never picked it up again. It sounds like the kind of thing a child came up with. Or, perhaps more appropriately, it sounds like the kind of thing a bad dungeon master would come up with to upset his players. Honestly, a good chunk of old school D&D monsters feel exactly like that.
* * *
- Displacer Beast? "Ooohh. It always appears a few feet from where it really is! That will confuse the players!"
- Beholder? "Super anti-wizard wizard! Shuts them down with an anti-magic raye and then shoots lots of magics at them! That'll show them!"
- Rust Monster? "I really regret letting Bob get that magic sword. This will solve the problem..."
- Mimic? "This will teach those players to go grabbing my treasure chests!"
- Trapper? "The floor as a monster!?! Super awesome!"
There is a really great series of articles written on the Internet that go through some of the ridiculous monsters and point out the inherent silliness of them, so I won't go through it. In general, the design philosophy of that era appeared to be a competitive Player vs. DM style. When considering the design style of some of those adventures (Tomb of Horrors, etc), it seems a consistent design philosophy.
As an adult role-playing gamer, I always focused on collaborative storytelling with some mechanical hurdles (combat, skill resolution, etc). The enemies in the story should have some theme or organization. They should feel like they're part of the story and they belong in the world. Whether it be an undead army or a clan of ferocious orcs, I expect to use a lot of the same kinds of basic enemies but with different skills. Honestly, I feel like an entire campaign could easily feature nothing but player races as enemies with basic animals for support. No flying balls with eyes or monsters that jump out of the floor, as the floor.
The design style of those ridiculous, old-school monsters feels out of place in the kind of adventures and campaigns that I want to run. In the kind of games I play, I find these kinds of ridiculous monsters jarring when they come up. It's a strange thing to go from fighting undead foot soldiers or the cult of a dragon god to coming across a pair of rust monsters in a dungeon. It takes away from what I enjoy in the game and it always concern me when people get excited about them.
This is my idea of Dungeons & Dragons.