I delight in running all levels of Dungeons & Dragons. There are always interesting challenges you can throw at the party, and they can always surprise you with their solutions.
I find nothing better than setting a challenge without knowing how to overcome it, and watch the players invent something in front of my eyes.
As the DM, you are the interpreter of the adventure environment. You should have a good idea where the key elements are, and then be able to extrapolate where additional elements are.
I've just been reminded that one of the most significant changes in 3E from what came before was to make it much harder for high-level monsters and characters to resist spells.
We're still living with those effects.
In AD&D, a very high-level fighter needed a 6 to save vs a fireball. There was nothing the caster could do to change that number (as I recall).
Add in magic (cloaks of resistance, rings of protection) and the fighter was only failing to save on a natural 1.
In the early days of Dungeons & Dragons, the XP tables were designed so that characters went from 1st to 9th level relatively quickly, and then slowed down.
Gygax suggested that it take 40-60 sessions (a year of play) to reach level 9, then a gain of 2-3 levels a year after that
Meanwhile, Hit Point acquisition slowed down significantly after "name" level. Magic-Users gained a solitary hit point per level thereafter!
However, the power of magic-users (number of spells, etc.) kept increasing significantly.
I think it's fair to say that the various designers of D&D during the early days weren't sure WHAT to do with the higher levels. There was a structure there, but most of the game was aimed at levels 1-9.
Trap silliness: in one of my dungeons, a group of kobolds lived in a room where they’d set up a swinging log to hit anyone entering the chamber. I think it was possible to detect, but my players didn’t, and so triggered it before being swarmed by kobolds.
The players survived the experience, and then, because players, painted a smiley face on the end of the log and reset the trap. Then left.
Several months later, they returned to the dungeon, some new players, some experienced, and looked down at their map upon which one room was marked with a smiley face. “Let’s go there, they said!”
Some traps are designed just for amusement value, though often of the DM not the players. (Though, given the right group, players can find them awesome as well).
One of my “favourites” is found in Castle of the Mad Archmage. It’s a stuck door that is actually made of balsa wood, so anyone trying to force it open hurtles through and is impaled on the spikes beyond.
It’s completely ridiculous and made explicitly to punish a common manoeuvre in D&D: opening a stuck door. That ridiculousness is why I’m so amused by it, and so have been more than a few players.