I’m sure I’m going to think of more stuff later but
Coolest shit:
-modrons! I love ‘em. They look cool and they’ve got good lore
-sigil, da city of doors baby greatest city in da planes. A good shape for a fantasy city (car tire), hotbed of cosmological intrigue, weird lookin buildings. Also: floating atop the Spire in the center of the Outlands but isn’t actually there, like atmospheric phenomena
-the factions. love a bunch of weirdos in a shadow war with each other and also serve some kind of municipal function. It is very funny to me that the design team got this idea from World of Darkness!
-belief shaping material reality in a very direct way, one of those ways being whole towns sliding into different dimensions because the alignment got too out of wack (literal vibe shift)
-the blood war! Some people think the blood war is stupid but I love it. two different groups of evil monsters are locked in an eternal war over who gets to control the multiverse and there are a multitude of other parties invested in keeping that conflict going? Cmonnnn
-rust dragons. answering the question ‘what if rust monsters, a creature everyone hates, were the larval stage of an even worse monster’
Dumbest shit:
-the “planar cant”. In the 90s ttrpgs suffered from something called “being written using entirely too much in-universe slang” and Planescape is no exception.
-schools of spells & magic items being more/less effective depending on which plane you’re on/how far away you are from your god’s home plane if you’re a cleric. This is easily ignorable especially if you’re not playing 2e but it all seems like a headache to track and not very fun!