at Borders as a kid, it might actually be the first non-core D&D book I ever got (it or the Complete Book of Dwarves). It was beautiful, and I loved the genasi, and aasimar (though my older brother had the Monstrous Compendium which introduced those), and tieflings, and it was just so many worlds full of wonder. In the back it had a glossary of planar slang (i.e. crazy jargon) and I kept thinking I recognized some. Wasn't till I was reading Chaucer in highschool that I knew for sure that yes I recognized some and that it was a large amount of Middle English (and as Planar Common is slightly Infernal influenced common this suggests that Infernal is Middle English ). The language of it, though, the slang that is, was just so neat as a kid, it really worked to create an atmosphere of foreigness, like this isn't just another place, but somewhere new and different.
I got Spelljammer books before the Planescape Box Set, though, and Spelljammer was my absolute favorite setting as a child, but as I matured Planescape moved to the front and has sort of stuck, I like that there's so much room to put in your own things, but there's still a good core to build off of.