I'm posting some minor questions here. It's stuff that's probably too small to need is own topic. Feel free to add your own.
Do portal keys disappear when used?
Is there an actual difference between planars and primes, besides their different places of origin?
Are the faction's world-views primarily centered on Sigil? Or do the Mercykillers, for example, depend on the Harmonium and the Fraternity of Order to enforce justice on other planes as well?
Here is another difference: theoreticaly planar PC can be summoned by Summon monster spells.
Yes, it does sound unusual but one entire entire adventure (In "Great Modron March" anthology) is based on that premise.
At lest that was back then in 2ed, suppose that was one of things that was removed from 3ed for simplicity sake along with cleric spell modifiers, planar weapon bonus and penalties, etc...
EDIT: Now to think of it, I think that planars were also weak against certain spells like: Banishment (wich sends them to their plane of birth) and spells that affect aligment.
I think Factions' views and agendas span the planes. Mercykillers don't rely on Harmonium and Guvners on other planes, and in fact they'd like for those other factions to just go away leave them to police The Cage themselves.
I'd argue that mortals are mortals and therefore material beings as long as they live, but since your statement is backed by a source I better pike that.
3e made the generic summon monster spells always summon outsiders or elementals, but planars can still be summoned in 3e, using any of the planar ally spells or gate, or perhaps the updated versions of more specific summoning spells. This is a big deal for planars because it makes it a bad idea to aggravate people with access to spells like commune because they could find out your true name and summon you up somewhere you really, really don't want to be. The various spells that specifically affect planar creatures also work on planars when they are not on their home plane, which could be really lousy if your PC was born in say, the Abyss.
As for the Factions, it depends. The 2e material emphasized that the Factions held the most influence, power, and presence within Sigil, in much the manner that say, the US Congress is way more notable in Washington D.C. than anywhere else (even though they rarely bother with anything directly to do with city affairs), but they are spread throughout. However, the Factions were clearly concentrated on various planes of influence and might have very little representation outside of them, especially for some of the smaller and less organized factions like the Bleakers. A number of Outer Planes, particularly those with homegrwon sects (ie. Mount Celestia) or very controlling natives (ie. Baator) do their best to keep the nose of the Factions out of everything. Factions seem to have more overall influence on planes with less formidable power groups (the aforementioned Bleakers in Pandemonium) or where they can set up standing arrangements with those present (ie. the Guvners on Mechanus). With an area of its plane of influence that it controls a faction pretty much has as much control overall all aspects of life as it can hold, and doesn't rely on anyone else's influence for help (the Mercykiller city of Vorkehan on Acheron is a good example). The situation in Sigil, where the Faction have each accepted certain aspects of civic life is both a political compromise and a reflection of Sigil's complete lack of any true municipal organizations. Most other planar cities, those not run by the Lady of Pain, have governments with rulers, city councils, police who get paid (albiet often in souls or the screams of their victims) and so forth. In those situations the Factions become more like philosophically oriented planewalkers associations, kind of like some combination between the Masons and the old British Royal Society.
Calmar wrote:
It is unusual, and "planar mortals" (in some way they had build in outsider and extraplanar subtypes before they were invented for 3e, without even have to wory about raising if they are slain) are not as powerfull as fiends or celestials but I guess that summoning party of experienced planar bashers is easier than dealing with devils, demons and angels.
Also they are resourcefull and well equiped individuals who are more sutiated for some jobs than some fiend.
Of course after spell end the dangers of having party of angry planewalkers screaming for your blood is on the same level as selling your soul to fiends.
One thing about planars and primes that no one has mentioned yet is that I believe only primes have silver threads when they enter the Astral plane. I can't remember where I read this or what the reasoning for it was, but I can only assume it was somewhere in The Guide to the Astral Plane.
Thanks Mechalich. That answer causes lots of new mysteries. If even the factions who are not actively recruiting concentrate themselves mainly on Sigil (like the Dustmen), how do they expect to alter the multiverse? I thought the factions to be the greatest powers on the outer planes after outsiders and powers and now it seems there are dubious sects that apparently possess more power or influence than certain factions (?). 
Yes, you're right. Pag. 8 under the voice "getting to the astral".
They don't explain why.
It was also in the Canon books. IIRC, the Players Guide discussed the differences between the two.
Planers could see portals, had no silver thread (not being latched to a Prime Material Plane, though whether they died in the Astral or just were returned routinely to their plane or origin wasn't specified in that book), and were summonable and banishable by spells in 2nd edition. Additionally, Protection from Evil type spells were also efficacious on Planers, i.e. No Touchie the Protected Dude. Holy Word and Exaction were also useful against Planers.
Which made things a bit more dicey for that vengence seeking party then they would prefer. Clueless Primes had their ignorance; Planers had thing weighed against them by their nature.
Factions benefited from their freedom in Sigil. Exactly how are the Mercykillers supposed to operate in the Abyss or Bytopia with a bunch of Powers to constrain their actions. So I'd guess most of the Faction Action (beyond mere philosophical membership) would be limited to the Outlands...
When planars see portals, do they only see a faint shimmering (or some other reminder of their presence) or can they see the portal's destination?
So would that mean planars would walk around Sigil and see half the doorways, windows, and other gates glowing?
Rofflestopheles wrote:
So would that mean planars would walk around Sigil and see half the doorways, windows, and other gates glowing?
Interesting questions.
In our campain planars can see portal but only if they are focused on such task (they want to find it).
Can they see portals destination? I think that depends on the portal itself. In 3e book "Manual of the planes" is given list of colours in wich portals glow depending of their destination (for example golden light for Mt. Celestia). So I guess the same coloured glow could be seen by planars when the portal is inactive.
So that colored haze gives clues about general destination where portal leads. Also I think that level of haze waries from portal to portal. (I think in one TSR adventure PC could see misty graveyard on the other side of city's Mourge portal).
No, I don't think they automatically get a shot at figuring the destination.
It needs to be coupled with a spell or skill to get a sense of where it goes IIRC. But it's your game. If you figured the portal at the end of the ally goes to the Elemental Plane of Fire and at some point the characters are pinned down by superior forces and want to jump...it's your call.
What is it with the abishai? They seem to be the primary force of Avernus, but I wonder if they are baatezu at all. Somewhere some berk told they are creations or servants of Tiamat, instead, that are just borrowed to Bel to fight in the Blood War. That sounds pretty poor, of course.
Well, they DO come in the colors of the five traditional chromatic dragons (in their original incarnation as well as in 3E, but not in their Planescape monster entry, where there are only 2 or 3 varieties, not 5). Their colors match those of Tiamat, they do look somewhat dragonlike, and Tiamat was originally portrayed as the ruler of Avernus before anybody ever heard of Bel (or Zariel!). So are they linked? - I'd say so! Maybe Tiamat is more involved with the Baatezu than generally thought... most berks seem to think she just HAPPENS to live in a cave in Avernus, they never take stock and wonder WHY... 
Might also have to do with Tiamat originally being a Lord of the Nine, until that was retconned to say she was never a Lord of the Nine, just, like, a gatekeeper or something.
The abishai are created by Tiamat from her share of the damned, those who worshiped her in life. As she has quite a following, she has plenty of them for her own purposes -- guarding her fortress, answering calls from mortal worshipers, and the like. The rest she is able to hire out to the baatezu as grunts in the Blood War. They're not the primary force of Avernus, but they're a good chunk of it.
(My source is Tyrants of the Nine Hells.)
Thanks. 
Can this Fiendish Codex be considered to work fine for Planescape? It seems to differ from the original stuff regarding the erinyes, at least, AFAIK.
I think both the Fiendish Codices work really well for Planescape for the most part. Especially the first one.
Yeah, I own both Fiendish Codices and they're both great stuff. FC2 could be a little clearer on whether obeisant acts are a problem for lawful good cutters, but other than that minor philosophical bobble I find it a great setting resource. I have to remind myself that the average PC doesn't know 90% of this, and knows another 5% of it wrong. :^)
I'll try and partially answer some of those:
Depends on the portal. Some portal keys are consumed when they are used, others are not. If it's not mentioned in the description of the portal then it's the DM's call, and really depends on how hard you want to make it to use a particular Portal.
Not sure on #2, but I know that those born on the planes can see portals whereas those born of the Prime cannot. Apart from general planar knowledge I wouldn't imagine there would be great differences between planars and primes of any given race.