In the "what eras would you like to see?" thread, Shiba Gunichi mentioned a period of time in second edition when anybody could become a shugenja just by studying. It made me think: what other setting details have been true in past editions that nowadays make you go "what the . . ."?
My personal favorite is when I went looking something up in Way of the Lion and discovered a whole section talking about how Kitsu shugenja regularly visit Jigoku. O_O That was how I learned that the cosmology of the Spirit Realms/afterlife/etc used to be VERY different from how it's presented in 4e . . .