While exploring YouTube comments lately I saw someone say that the old Planescape setting (which in 5th edition has been posthumously grafted into the Forgotten Realms for the most part) was "insanity." I got into DND in the 5th edition, but I'm pretty familiar with old Planescape content (I've done lots of research preparing to write things for the DMs Guild that I have since learned are not guild legal - see, "the most part") and wouldn't describe it as insanity. No more than Baldur's Gate: Decent into Avernus, or Waterdeep: Dragon Heist.

So for all y'all folks out there who are more experienced than I. What makes Planescape insane, and what made it feel different from the other campaign settings of the time?

I don't remember anything especially interesting mechanically.  Sigil was a city that could accommodate a lot of different ideas at once.  The Planes were a great adventure backdrop.  The Factions were kind of a first for D&D, although Planescape was cribbing them from Vampire: The Masquerade.  Tony di Terlizzi's art was very unique and had a lot more flavor than most of the stuff you see today.  The goofy faux-Oliver Twist slang the books used got on my last nerve, but some people seemed to like it.  IIRC, it was the last setting of 2e, so it had a kind of end-of-the-party feel, but maybe that's just me.

Overall, nothing inherently insane about it, but plenty of room for insanity in the execution.  Maybe this guy just had a good experience with it.

Mostly in flavor and Descent into Avernus borrows heavily on Planescape.

Sigil has literally everything including a god you better not acknowledge as a god or she'll kill you.  Has portals into literally everywhere in existence if you can figure them out.  

Those portals also let DMs screw with people.  Because every door and archway was technically a portal somewhere.  But who knows what the key is.  Might be an object, might be thinking about the word Pumpernickel on a Wednesday of the Toril Calendar.  So if you wanted a break as DM poof, players are somewhere else and have to figure out how to get back.

Thank you. Those are very helpful answers.

1.) Very unusual art, but in a good way

2.) I mean look at Sigil. "Literally everything including a god you better not acknowledge as a god or she'll kill you."

Thankfully the portals were almost always two way, and the key to go through one almost always worked the same way back. So with very few exceptions, if you could get through one way, you can get through back.

The Lady of Pain!
Lovely woman, very nice.

Yeah, the keys are forgiving or nightmares depending on the DM.  

 

If the key was a physical object no problem.  The problem arises when there's time links or thoughts involved.

If the key is a certain song a failed perception check might mean no one thinks about what the bard was idly humming as you guys walked under that bridge and ended up in Mechanus.

Planescape wasn't insane, it merely just turned most of the typical high-fantasy tropes common in previous settings like the Forgotten Realms, Dragonlance, and Greyhawk on their heads. Alignment was far less important, politics was a thing, murderhoboing was significantly discouraged, and PCs were often forced to peacefully interact with beings that were traditionally just monsters that had no personalities beyond "rawr, I kill you mortal." And also, there was Planescape: Torment, which is still one of the best CRPGs ever made.