As a kid, I always felt that to qualify as a land, an area had to represent multiple locations. Since Mainstreet seems to represent just one location, as does Toontown, it's not really a land. Disneyland is a land made up of other lands, so it qualifies.
If you picture Disneyland as Walt left it, it's very strong in the concept of Realms or Lands being comprised of multiple related themes. As you enter Fantasyland, you are in a castle, but quickly find yourself in Neverland looking at Skull Rock. As you enter Adventureland, you are in Tahiti, but quickly find yourself in Africa. As you enter Frontierland, you are in a Fort, but quickly find yourself in New Orleans or Rainbow Ridge. There was always something magical about how Walt and his crew made such a complex jumble of themes work so well.