What changes do you make from canon and how is it done? What's the results of your changes and how have they worked out for you? Everyone makes their games alittle different from canon, so it's interesting to hear how others have managed to alter their world a bit.
Feel free to comment on my changes and I'll do the same.
I'll post mine in a bit.
I ignore alignment. It's an overly simplistic concept and is as much a hindrance to story telling as it is an aid. Plus no one can agree on what the alignments mean anyway. Besides, you don't need to know that Celestia is "Lawful Good" to know what it's about. Otherwise, I go with canon. It's never served me wrong. That doesn't mean I won't heinously violate the canon of any other settings my PCs wander through, however. 
So far, the only changes to cannon I've made have been inadvertant. 
I don't have the 2e books...yet.
I steam it up a bit. Not quite to the degree of being a neo-Victorian gadgeteer's dream, but a bit more clockwork and airships never hurt anyone.
I had a different take on the steampunky bits, actually. Somewhere, in one of the books, I read that wood is more expensive than stone and metal in Sigil, since wood is more difficult to magically create and has no elemental plane to mine it from directly. I went a little over the top with that and made a Sigil were only the most wealthy can afford wooden doors or floors. I also included very, very, very primitive gun powder technology, of the kind were a weapon is nearly ten feet long and needs half a minute to reload. Most people just laugh about it. Airships aren't in yet, but might come later, I have a gnome in my group who is a crazy inventor.
Me and my gang are new to the setting, but even atthis early stage...
Deep Limbo - All the information and monsters of the Far Realm have become a part of Limbo. It makes the Plane much interesting and dangerous.
Steam, Clockwork, Gas and Magic - With countless Material Planes housing countless inventors, Sigil is full of half-ideas and strange contraptions powered by everything from dirt to souls. It makes the players want to search for items, its better than giving them longswords+1 anyway.
is Insanity not this backwards speak - Slaadi and chaotic related people don't talk funny. Madness can be better represented than that.
The Astral is NOT connected to everything.
It connects only the Outer Planes and the Prime.
The Ethereal connects the Inner and the Prime.
2e Cosmology at its finest.
Thats one of the ones my groups stuck with too. We didn't use power keys and the like though (a little too complex for players new to the setting).
As I think about this, here is a few things that are slowly creeping in,
Tenebrous? No.
Athena is now head of her panthen because we don't like Zues. Zues is CN.
A lot more goblins live in Sigil, acting much like Victorian underclass.
Slaad and Modrons are not the outright foes the Upper Planar races and the Lower Plana races are. Slaad think Modrons are just a different kind of chaos and the Modrons think Slaad could be classifide given time.
One of my favorite sourcebooks is Beyond Countless Doorways, a d20 title by Cook.
One of the most knowledgable planewalking NPCs in my game argues that the Great Ring/ Prime/ Inner Plane cosmology is basicly screed. Planes are just planes, or rather what planars think of as planes are in fact collections of 'indeterminately finite' realms in perminant conjunction. The 'plane of fire' is in fact many connected fire realms/planes. The Great Ring Works for people because it is a way of having a mental 'map' of several well-known routes of conjunctions, but that's all it really is, and it works to think of the multiverse as being a tree, or a mountain, or nothing at all.
I have no problem introducing tech into my Sigil - so guns, of many sorts, are not unheard of; neither, for that matter, are railguns, plenty of steam powered contraptions, including steam-mechs, and the very rare laser pistol.
The problem is, anything more complex than a six shooter or steam tech tends to be very unreliable in the planes, so sci-fi level technology is mostly a curiosity rarely seen in practice.
In fact, the story line I used to bring the Drow back to attention in my game (I've not even had a drow in one of my games for 7 years or more; much less used them as the cliche villain - but, like Romulans, I think that every once in a great while they need to remind people why they are scary) involved sneaking a nuclear warhead into Sigil. (though that was really only the cherry on top of a very complex and devious string of events)
Usually, though, magic and bizarre tech (chaos tech, soul powered tech, etc. Have a building called the "think tank" in the lower ward where people are tied into machines and their creativity is harnessed as a means of power) are more common to the story.
I completely ignore Faction War.
Don't use spell keys - as I don't think my players would be keen on the idea. (though I've been gradually introducing them in a way to make them seem cool)
That's about everything off the top of my head ... I've found that I like the setting (and have more luck with it) as written in most situations - only changing things when I feel the story demands it. (or to make it a little more "mine")
For awhile I went through a string of alternate approaches to PS - sci-fi, cyber punk, other science heavy variations - but never really got comfortable with it until I stopped fighting the material. (and PS:Torment really helped me understand the setting better)
Yea, I have high-tech stuff in my campaign too, but it runs into problems on highly magical planes. In fact, one main PC is a Psion from a sci-fi world who accidently stepped into Sigil, and he has, in a more recent adventure, even been to Urban Planescape Sigil.
Also, as far as the faction war goes (no I never bought the adventure), in my campaign, it "sorta happened". The Lady's demand to abandon the factions was in fact instantaneous not lingering, and factioneers only took about a week to filter back in. Nobody talks about it, and it is very bad manners to mention it, because, in fact "(#*)It Happens" from time to time. It wasn't the first Faction War, and unfortunately probably won't be the last. Nobody really wants it to happen again, except the Anarchists, since the Factions lost some power and the Sects are on the rise. So the Sons of Mercy + Sodkillers = Mercykillers, and they still run the prison, but the courts and the hall of records are merely dominated by the Guvners and Takers, respectively.
1. The tech level of the planes is on average what Alternity and D20 Modern define as PL 4, roughly the equivalent of 19th century earth. Some more advanced places exist (some worlds may be in the 1920's or beyond), but they're hard to reach.
2. The Ethereal Plane and The Shadow Plane connects to just about every plane in the great ring.
3. The Astral plane also does.
4. There's the Spirit Plane, and the Fey Realm, Plane of Mirrors, Far Realm, and Dream Plane and they also connect to a bunch of other places.
5. The planes are hard to map, there are things out there not quite accounted for in many cosmologies.
6. The faction war happened 80 years ago, and the Lady's decree still stands, even though many of the factions and others are slipping back in to the Cage.
Does anyone not ignore the Faction War?
My players aren't that into the details of Planescape, so I haven't made many changes. The one with the most impact is the advent of "Alignment Light" rules. In my universe, it is easy and common to disguise your alignment. Hence, not being LG does not preclude your ability to visit Mount Celestia, for example.
Of course, everyone who visits the Lower Planes disguises their alignments. :mrgreen:
I personally love the Faction War. It keeps the factions from totally dominating Sigil.
I love the sects, and I like the idea of new philosophies gaining more power. In my PSCS games, I've featured the Disposessed, Ring-Givers, the Illuminated, and have plans on introducing the Planewalkers Guild and the Infinite Staircse. I also like the old Factions, but they are more splintered in my sort-of-post-FW Sigil. The PCs have contacts with a sect of Sinkers who try to keep things from going to entropy too fast. Mercykillers practicly run the prison, but the Sodkillers and the Sons of Mercy have different agendas and methods.
Still, all the big Factions have representitives on the Sigil Advisory Council, but so do Guild interests, semi-autonomus local districts, and various others. The only remaining legacy of FW is that they're still arguing over who's going to collect taxes in the Birdcage, because as a Faction, the Fated decided it wasn't worth it anymore. Though individual Takers may still have jobs as property lawyers, assessors, record keepers, etc., there are more Guvners in the Hall of Records, Fated in the Courts and Prison, Athar in the Foundry and Armory, and so on and so forth.
I don't know, I think that's sort of the point. The irony being that the Factions dominate Sigil but actually have less power than a normal government since dozens of competing groups control vital industries.
My own groups went with post- Faction War. It gave an added layer of intrigue as the Factions now have to work through tools and proxies in the City of Doors. And it let non-faction groups take some stage time too. We now run into the factions more often in the planes as the factions have had to refocus to other territories, and can fight each other more openly for turf.
So as far as politics in my group's game - they just got even *more* complex in the City and on the planes.
I retain the 2e Astral and Ethereal roles, but I use Shadow as a full plane per 3e.
As for changes... hooo boy...
Mystra's death at the hands of Karsus' Avatar spell wasn't exactly as is commonly thought. Karsus had help.
Elysium lost the layer of Belarian, which was ripped from its metaphysical moorings and dragged briefly into the Waste to become the 4th Gloom. Eventually it was spat back out and ended up as a domain within the Outlands, with a half-dozen groups and races fighting over its fate.
The General of Gehenna and the Crawling City vanish from the planes
Mydianchlarus and Anthraxus are ever so dead, along with Typhus, Bubonix, and Cholerix.
Big, big, BIG death toll on the part of the 'loths and the guardinals
I use or ignore the Faction War as the plot demands for each game.
Power and Spell keys are just plain out, and I try to simplify the effects of the planes on magic a bit.
I've only used it in one campaign (although it will be used again) Orcus failed to return from the dead in Dead Gods, and once he did return, without his wand, he had only a fraction of his power and was being driven insane by the taint of law left in him from his stint as Primus. So he cut a deal with the Baerns to stop the pain and get him back his power. Only they drove the last bits of his mortal soul and all of the chaos in him out with the law using the Heart of Darkness, effectively making him a arch-yugoloth. That forced Thanatos slide over to Carceri to become a new orb, Orcus, once again calling himself Tenebrous, slew Apomps at his new masters command and took control, and finished, the Tower of Incarnate Pain.
Drake
Here's a list of my customizations for my Planescape Campaign.
1. Sigil and the Gate-Towns are actually their own Demiplane: It's a transitive Plane between the Astral and Ethereal Planes rather than dwelling in the Outer Planes. Furthermore, there's absolutely no gods on the Demiplane but the Lady of Pane. Amongst Sages looking to be original, it's called the Demiplane of Portals.
2. Faction War never took place: It just didn't seem to be something that would interest me all that much.
3. The Transcendent Order is completely different: The Transcendent Order, in my games, seeks perfection rather than action without thought. Thus, they're constantly trying to seek to improve their skills and ability above all other things. They're the Monk and "Raistlin Majere" mage Faction.
4. The Signers are a bit less existentialist: I tend to make them more like Mage: the Ascension in that they're aware WILL creates the cosmos but don't necessarilly believe it's their will ALONE. They're still insufferably arrogant berks who treat the 'Unenlightened' with contempt.
5. The Multiverse is implied to be a lot bigger than people suspect: There's even occasional hints that The Great Wheel is just a gigantic simplification for a vast horde of completely unrelated but linked together planes with Godly Divine Realms not actually part of the planes they're linked to at all.
6. Sigil Warps Fiendish Minds: Fiends & Archons, in my game, don't have free will. However, that changes in Sigil where they can actually shift their alignment with great amounts of effort. This is why a lot of true demons and powerful ones avoid it almost as much as the Upper Planes. Archons also find it extremely discomforting.
7. Nirvana replaces Mechaus: Gears and Cogs land is only a layer of it.