I have no idea where to put this so I'll put it here. Like to title says. What is it that defines Planescape for you. What is the most important thing about it that makes it special, or just that makes you like it.
What is it that defines Planescape more than anything else?
Great! That's about what I thought, though I couldn't really put it into words. Now I can just steal yours!
Philosophers with clubs
Doubly true when it comes to the Harmonium and Mercykillers.
For me it's tourism. Any given Prime has some fascinating places and cool people, sure, but you can't see the Spire or Nimicri the Orb-Town or Neth or a floating river of fire or... that sort of thing, anyway. :^)
Approaches to conflict that do not necessarily involve combat. (You don't just beat up every Abyssal Lord you encounter and steal his stuff.) A great deal of pre-Planescape material did not focus very much on this kind of RP. The idea of having lots of ideological groups or factions with conflicting goals (other than Heroic Hero and Evil Bad Guys) was pretty well developed in Planescape compared to earlier stuff.
Of course, I went too far in the "not hack and slash" category with one of my Planescape characters.
Another thing I like about Planescape is "high weirdness," though it's present in some other settings as well (like Spelljammer!). Githyanki are weird. Githzerai are weird too. Githyanki have a lot in common with various historical human societies, while at the same time they're decidedly different.
Of course, mind flayers in spelljamming ships are also weird, as are giff and scro. (Who can not like "civilized," intelligent orcs who speak fluent Elvish?)
BoGr Guide to Missile Combat:
1) Equip a bow or crossbow.
2) Roll a natural 1 on d20.
3) ?????
4) Profit!
For me it's mostly about philosophy and exploration. Also, weirdness and non-combat resolution to problems is cool, too.
Pants of the North!
For me, Planescape is a place where ideas are made flesh and the pen truly is mightier than the sword. It's a setting where the battle of philosophies becomes a literal thing, where knowledge really is power. I love that the setting takes ideas seriously enough that the very shape of the multiverse is a reflection of their nature. Planescape actually would have you think about the deep questions, ponder what life is about and actually have a chance of taking something away from that. This is a setting that puts the creativity of players and GMs at center stage. I think it is the one setting I've ever encountered that actually elevates roleplaying from the enjoyably frivolous into the realm of actual art.
Our group has sat down (well, over email) and tried to hash out what the core things that make 'Planescapeyness' a lot of times. I think we managed to narrow it down to:
1. Philosophers with clubs.
2. Whimsy.
3. Cynicism.
Cyncism and whimsy are a strange but potent mix. Philosophy makes the Planes go 'round and gives us stuff to do amidst all the cynicism and the whimsy.