I encountered Planescape not long after being exposed to D&D itself, and it was the first time I encountered anything involving steampunk (even if only in light doses), the idea of a fantasy universe that wasn't obviously LOTR derived, or the idea of demons as having a deeper agenda than Kill 'Em All. I will always be grateful to it for giving my young developing mind the idea of outlandish settings, over-the-top and outright alien fantasy locations, of "evil" being more interesting than just something else to kill, and that fantasy doesn't always have to be human-dwarf-elf all the time. I met Mage: The Ascension almost immediately afterward, and so philosophy as a weapon was a very powerful idea for me for a very long time.
I honestly haven't returned to Planescape in a long time, so I'm not sure how it's held up. As I no longer care for Alignment systems of any kind (3E, 4E, or otherwise), I imagine I'd probably streamline the Outer Planes a bit, as a few felt slightly redundant in retrospect. I might also try to include a mechanical way to incorporate "belief has power," which was heavily emphasized in the fluff...but not at all in the crunch. And I think that perhaps the not-quite steampunk aesthetic might benefit from either just going all out and embrace that genre or abandon it altogether.
I still am a little sad that WOTC never updated or allowed anyone else to update Planescape in 3E like they did for Ravenloft and Dragonlance...