Mostly it's the unique take on fantasy that makes it so great. In my opinion Planescape is one of the best RPG worlds, second only to Morrowind.
I played Morrowind a few hours, years ago, and I thought it was boring as hell. Then I played Oblivion, almost until the end, and I was bored out of my mind. Didn't even fight the final boss as I didn't care.
Actually I am seeing a little bit of PT 's ending on youtube and I am quite struck by how philosophical and intelligent is the story. Morrowind has no story as far as I know.
*****************SPOILER'S ALERT, IF YOU ARE PLAYING PLANESCAPE TORMENT, DO NOT KEEP READING UNLESS YOU WANT TO 'RUIN' THE GAMEPLAY**********
just saw a bit of the part where the 'immortal spirit part' of the Nameless One is negotiating with the Nameless One. Even though I did not meet the characters that died, I was struck about the fact that they died to help the Nameless One.
Then I stopped watching. Maybe I'll play it, but how many hours does it take to get to the end? gotta decide :)