Is it the sheer number of available rp options? (Anything from a human street urchin to a baatezu exiled from infernal court.)
Is it the limitless possibility of really weird stuff? (Anything from a dungeon delve in the Far Realm to politics in the City of Brass.)
Is it not PS itself, merely that some of us get sick of the traditional D&D-isms, and PS offers something different and jaded? (Grim-n-gritty factions instead of campy alignments, steampunk/horror/intrigue instead of vanilla fantasy.)
Is it the fact that PCs are merely a "drop in the ocean", no matter how high-up they become?
Is it because playing a PS character puts us "in the know"? (Other campaigns, and real life, focus on the "little pictures" while we PS players feel special because we can see the big picture.)
Or is it just DiTerlizzi's incredible artwork?
TS
Me, personally, to me it's all about the atmosphere and the possibilities. The other planes being so easily accessible makes for some interesting quests. The mystery of the setting also adds to the alure I believe. I know the big thing that got me into the setting was how different it was. I just got fed up with the usual stuff. Dragons, castles, unstoppable mages, Drow, kings and kingdoms just got so boring when it's all you ever see for...ten plus years. In PS the only real thing that was unstoppable is the Lady...and any attempts to prove otherwise are always a good laugh at the person who came up with it's expecnce. It's just so out of the norm and brings the things that are usually concidered "too high up" for traditional settings like Forgotten Realms and Greyhawk players and DMs to deal with. As you've stated you have Baatezu and humans interacting...while not always peacefully..but then again the multiverse isnt about peace. It also opens alot of players eyes, usualy they are just concerned with just that little continnent they are on or that small region of the area like the Spine of the World in Faerun, PS introduces so much more. It's kind of like how we all used to jsut be concenrtrated on Earth then we hit space and were amazed by just how much more is out there, That's how it was for me when i first picked up PS anyways. It's just the sheer ammount of possibilities. No one PC will ever experience everything there is about planescape and I love that! When you hit epic levels it's pretty much boring cause you've seen it all but you can be level 300 in a planescape setting and still get inked in the dead books *cough* Blood War! *cough*