"Planescape" is going to be relaunched soon: tomorrow for those who pay for the privilege of DnD Beyond, in a week or two for the rest. Before it happens, and "Planeverse" replaces ADnD sourcebooks as a New and Shiny Thing in a common perception, I want to memorize my own understanding of the setting.
So in a sense this article is somewhat of a personal memory knot.

To me "Planescape" as a setting and Sigil as a city-state is explicitly anti-establishment, "punk" in the most direct and sincere sense, and it is not only so because of the spiked collars and mohawks painted by DiTerlizzi but because of how it portrays those who have ultimate power, those who hold on the whatever power they can get and whose who are powerless. Unlike many modern/indie settings that over-boldly present their main message or the mainstream settings which are similarly very straightforward even if they are too cautious to have any message whatsoever, Planescape, in my opinion is a little more subtle, its themes a little more hidden than proclaimed in neat big statements. Yes, you can play it just as funny romp across the fabulously exotic planes disregarding the underlying themes entirely without changing a thing, yet I am still sometimes amazed how much of that was published in this officially-done-by-the-corporation setting, and I am still wondering how little of this attitude is going to see the light of the day tomorrow.
Why "The Athar"?
When I started DMing not everybody in my group could fluently read English so I started to translate "Planescape" in order to give people some material to read on their own and to avoid explaining the same things again and again. And one of the very first questions that lodged in my mind when I started to translate Factions, was "Why did they use this strange word for Defiers? Why 'The Athar' (the word that my then-paper dictionary didn't even know), why not 'The Lost'? Every other faction is using an understandably English or English-like word, even Xaositecs, so why Defiers are specifically so differently named?"
I cannot read developers' mind, of course, but much later, when I studied graphic design and marketing, there was a rule of thumb taught to us that in any given list you want to put your most important points first, as the first positions on the list are going to have the biggest impact and are going to be the most retained in memory. If so, it might be that Defiers were called The Athar to place them in a top of alphabetical list of the factions. Because what Athar as a faction says is: "See those beings that in any world are holding the ultimate power? The ones who potentially control your whole life and, most importantly, almost always control your afterlife? The ones who call themselves gods but as fallible as any berk? We don't think them important enough to worship and fear, despite their power."
In many settings the gods are the ultimate beings of power. God-murder became somewhat passé in recent years, especially among OSR settings and micro-settings which are littered with god-corpses, but in times of ADnD, I believe, it was still a rather rare and unique event when a party could actually challenge a god even through their killable avatar, and many gods were not even statted. And here "Planescape" proclaims, through the alphabetically-placed-first The Athar, from the very first paragraph on major philosophies of Sigil, that ultimate powers who control vast swathes of the universe are not really any different from you or me except the scale of their powers, and you can easily regard all of them as frauds and bullies lacking of anything truly divine, if you wish so.
Was this trick of naming true or not, to me reading about The Athar the first from all the factions remains a powerful moment that sets the tone for the big portion of the setting. Punk attitude is a full contempt to any power structure and who is more powerful across the universe than gods? Yet, the disregard for them is positioned front and centre.
The underlay of Sigil
These are the tidbits about the Sigil that are stuck in my head the most. Obviously the Sigil is much bigger and more vibrant than this, but these moments are to me the anchors that hold my understanding of the city together, a canvas with everything else painted on the top.
• The Hive takes almost full quarter of the map; and if you add The Lower Ward to it, it is good 1/3 to 1/2 of the terrain given to the desperate people and unsafe living conditions.
• Hive slowly but surely grows all the time; there was an attempt to wall off the Hive away from the eyes of the proper folk but no wall could contain the spreading despair. There is a serial killer in the Hive, but nobody cares enough or is powerful enough to stop him.
• From 'Planescape The Torment' game there was this one clerk, from arguably middle-class and relatively well off, who was hiding on Great Bazaar for a nap because his boss was overworking him.
• People can hold portals for air and drinkable water hostage with a right key.
• Hall of Records was a former college which The Taken foreclosed on a technicality because they wanted the place and could do that.
• The Lady's Ward, the seat of most of city powers, is cold and sterile place.
Those who are in power
The most prominent anti-establishment aspect in Sigil to me is how the city structures of power – those who hold, use and abuse the power over the city as long as they don't cross The Lady – are portrayed.
• Judges who enforce the laws are Fraternity of Order; the best thing that can be said about them that they do sincerely adhere to laws (with an eye always open to spot a caveat not covered by them), but their rule is always shown as cold and distant, playing no heed to anything but the law itself.
• Cops/Guards are Harmonium: a faction that sees itself as righteous enforcers of good and order, but in fact are militant enforcers for homogeneity, rigid social order and complacency under their banner, allowing no free thought or having any acceptance of other ideas. There is a hidden faction within this faction that beats and kills 'troublemakers' such as Indeps when they can get away with it.
• Enforcers and executors are Mercikillers, a terrible mess of a faction with a zealous belief in absolute and ultimate justice and their right to enforce it by any means possible while themselves suffering no consequences for their actions as they claim to answer to 'the higher law'.
These three together with the Takers below create major governing structure which is cold, merciless, pushing for complacency and acting by the force.
• Arsenal with all its weapons is held by what is, basically, the doomsday cult.
• Hall of the Speakers, where laws are made, is run (rather ineffectually, IMHO) by The Sign of One people literally believing themselves to the be center of the universe. Some factions don't even have opportunity to present their cases, and at the best Hall of the Speakers serves as a release valve to divert some potential violence into shouting at each other from the podiums.
• And taxes are collected by The Takers, 'the might makes right' faction which is probably more hated than Red Death and Harmonium combined. To note, the maintenance of the city is provided by dabus courtesy to The Lady, so to me it always looks like The Takers decided to collect the taxes simply because it ingrains them to the other existing power structures such as Fraternity and Harmonium but mostly because they simply can take such things from the others and nobody can stop them.
None of these factions are portrayed sympathetically in their role as city-runners. I don't expect to read minutes from the meeting where Bleak Cabal asks for soup kitchen budget but I always got an impression that none of the factions in a position of power do anything for the benefit of anybody else but themselves, and while they run Sigil in a shaky alliance of mutual benefits and counterweights there is nothing glorious or worth of respect for any of them, just as it is with gods.
On the opposite end we have:
• Athar, who are portrayed as mostly powerless and/or harmless, preferring to fight their ideological fight with rhetorics and pamphlets.
• Indeps, reasonably anarchic faction who were decimated about 600 years ago by a plague which targeted specifically their faction (as it became too big) and are currently hunted by Harmonium, with their Ward slowly shrinking.
• Bleak Cabal, the nihilistic faction who is the most kind to the poor and forsaken of the city.
• Dustmen, a death cult who is probably the second-most charitable faction of the city, even if it is a long step away from The Bleak Cabal.
• Revolutionary League (rather cowardly, IMHO, renamed Hands of Havoc in re-launch) who are so deeply encrypted even against themselves that their fight is never-ending and rarely effective to enforce any major change (if it wasn't so The Takers wouldn't be where they are now.)
• Xaositecs, who are more flash-mob than a faction; while they represent an important aspect of the Planescape, they rarely affect the life of the Sigil in any significant way.
Expectations
Looking at those vibrant recruitment posters, MtG-like example illustrations and much softer DiTerlizzi's art I am expecting the grim and gritty aspects of the setting to be glossed over or downplayed in favour of 'Into the Spiderverse'-like appearances and mood. While previously Planescape could have been played this way but more grounded approach was just as valid, I am expecting vibrant and bright to be the only available approach for re-launch.
I am expecting the factions in a position of power (if any are to be in a position of power) to be portrayed much more softly, and the whole governance of Sigil mostly devoid of hopelessness and despair it had in the first Planescape.
I expect The Hive to be barely mentioned and/or being much smaller.
I expect Blood War (which is sometimes derided, but to me is an absolutely essential part of the Planescape) to be glossed over, be miraculously over, given just a lip service or never happen in a first place.
I expect the gods to be put back on their pedestals despite Athar still being present.
I expect infinite planes of New Planescape to be even more milquetoast and boring than the infinite planes of old Planescape; I think that human mind cannot truly embrace the infinity, but as planes have to be infinite for the sake of unbound fantastic universe, the farther we go from Sigil the less meaning all other places have, and I would be very surprised if any of the new or relaunched locations are any more interesting than the old ones.