If you like reading China Mieville or Neil Gaman or Riverworld, you will like Planescape. It's "fantasy taken to the edge," with a focus on wondrous planar locations, eccentric NPCs, and ethical dilemmas. There's no nondescript village to start in or save; in fact, most adventures start in Sigil, the City of Doors, an extraplanar megapolis with a jaded worldliness and war-in-the-shadows that feels like a blend between Babylon 5 and Casablanca. Yet, despite subverting fantasy tropes, it manages to remain relatable to players with whimsical humor and a cohesive worldly aesthetic. Opposition can come in many forms from fiends and celestials, to githyanki and slaad, to any number of factions with philosophical stances the PCs can support or reject. However, what matters most in Planescape is the *why.* Why prevent a gate-town from being dragged into Limbo? Why bring evidence to a falsely accused angel? Why plant a rose in the Abyss? Most of all, Planescape encourages the DM and players to look deeper than "rampaging orcs" and draw forth narrative themes to enhance their games, and maybe even make the players think about what they believe. While other settings may involve jaunts to the planes, Planescape is about finding meaning in the multiverse.
