My primary issue with Planescape, and I know this will not go over well here, is it advocated a view of the game in which setting material became important for its own sake rather than as a tool to create an interesting adventures. Particularly galling to me was the notion that it connected every setting, and that Sigil was the true center of the multiverse. Add in elements like the Lady of Pain, raising Demon Lords to the heights of deities, etc. All together it was as deprotagonizing as early Vampire.
I already talked about how PC was part of 2e's broader marketing strategy of "campaign settings for everything" which was bad for the company no matter how much good stuff it presented. Trying to connect every single setting was probably a mistake too; it didn't need connection to anything more than Greyhawk or FR, and only those because those settings both used the core AD&D cosmology, and because FR already had the assumption that it was connected to Greyhawk through the planes.
The Lady of Pain is there as a logical foundation for the setting: if Sigil has all these portals, then why isn't it controled by gods or fiends, or why isn't there a big cosmic war for control of the city? She really shouldn't be used by the DM at all, and players who are "let's kill the Lady and take over Sigil" are kind of missing the point.
And I'm pretty sure at least some elements of the settings (i.e., factions) were a reaction to Vampire too, though deprotagonizing was going on through a good chunk of 2e (*cough* Time of Troubles*)
My personal experience was that is was very playable, though. The whole setting was laced with adventure hooks, "planewalkers" were the equivalent of the "adventurer" sub-culture, there were low threat/high threat zones, and travel could be instantaneous (side quest to get portal key) or require the use of a planar pathway (side quest to traverse the River Styx, Yggdrasil, or Infinite Staircase).
I'd argue for its playability, but definitely not for it universality.
That's always been one of the things I though was good about the setting. The planes aren't just a bunch of high-level dungeons, it shows DMs how you can have lower-level characters interact with the planes. And there were adventure hooks all over the books here and there for the DMs to ignore or use as they saw fit.
Basically, Athas's isolation is grossly overstated.
I'd attribute that to 2e's rather conservative approach to PC power. The edition liked waving around the nerf stick a lot it seemed, and Dark Sun PCs started at level 3 and had an ability score range of 5-20 IIRC. The way 2e liked to approach things, that was overpowered munchkining to the max. So PS continued the whole bit about "Athas is damn hard to get to bit". In Spelljammer this might matter because an Athasian PC is probably stronger compared to someone from Oerth, Toril, or Krynn, but this shouldn't matter at all in PS where the game is supposed to be about more than just physical might. Though it went the other way too, I mean if you're in a Spelljammer or Planescape game, you don't necessarily want things like the PCs making themselves insanely rich by importing metals or water to Athas either.
