edbonny said:
Let's make a campaign setting by picking out the archetypal things about these worlds.
Uhm... Planescape has the whole belief theme, exemplified by the factions, as well as the concept that real power comes from knowledge. Another important element is that you can't always solve things through violence, because there's always someone who can beat you, exemplified by the ease of finding and meeting extremely powerful creatures. More specific items are the Lady of Pain, portals, the City of Doors, the exotic planes, the various factions, the Blood War, the cant.
Dragonlance is big on heroism; few people making a big difference. Great armies waging epic wars and conquering continents. The need to have faith in the gods even when they seem to abandon you. Distrust of arcane magic, leading to secluded wizards and uncommon magic. The problems of racism are also quite important - the dwarves and elves keep screwing up again and again mostly because of this (ok, so that's speciesism). More details include the best-known gods, the Orders of High Sorcery, the dragons and their allegiances, the Cataclysm and all the old ruins scattered in the world, the Test, the draconians.
Ravenloft, well, is about horror. Can't be much more specific; each domain has its own flavor of horror. Terror, insanity, no justice, no escape, only the choice between a short life of fighting darkness or a slow descent into it. Specific items include more dangerous magic, Dark Gifts, magical healing of curses and disease is hard to impossible, classical monsters such as vampires, witches and werewolves are made more powerful.
Forgotten Realms... uhm, I haven't quite played it enough outside of the CRPGs. Still - high magic, high levels, ancient empires of immense power, secretive organizations, the Underdark, Waterdeep.