Favorite Revolutionary Game Mechanic.
In AD&D races were the same as classes. Meaning one could either play a wizard or an elf. Now you can play as an Elven Wizard. The game has only gotten better since this change. Allowing players to make very interesting and fun characters that shape their gaming world. This trend is now in nearly all RPGs. Including Skyrim, Pathfinder, Dungeons and Dragons, and many others.