It was the first MMO to kick off the whole genre, truly. At least here in the USA, back in the time of 1997. It had no levels; just skills to hone. It had housing (later they they let you design it) you could literally log in and just be what you wanted to be, no questions asked - just go grind it. Want to be a miner? Go grab a pick and hack at a wall. Want to kill? Go to a dungeon. Want to tradeskill? Go hack down a tree for wood and turn it into furniture or bows. There was so much customization.