"AS I said previously, the concept of a half built but open to the public theme park started with MGM."
Actually I'm really not at all sure who started this concept, but as for the Disney company it started with
Disneyland.
On opening day there was:
MAIN STREET
Santa Fe & Diseyland Railroad
Disneyland Street Railway (the horse-drawn trolleys on Main Street)
Disneyland Fire Department (that fire wagon in the fire house was a ride up and down Main Street)
Disneyland Penny Arcade and Shooting Gallery
Main Street Cinema
ADVENTURELAND
Explorer's Boat Ride (now the Jungle Cruise)
FRONTIERLAND
Log Fort and Block Houses (actually, the entry fascade to Frontierland)
Mark Twain River Boat
Painted Desert (Mule Pack, Stage Coach Ride - Conestoga Wagons not open yet)
Davy Crockett Museum / Arcade (then the Mercantile)
FANTASYLAND
Sleeping Beauty Castle (the walk-through wasn't open for two more years)
Peter Pan
Mr. Toad's Wild Ride
Snow White Ride-Thru
King Arthur Carousel
Mad Tea Party
Canal Boats of the World (now Storybook Land Canal Boats)
Tomorrowland
Autopia
Clock of the World (This was a large clock that marked the entrance to Tommorowland, counting down until 1986 Haley's Comit)
Circarama, U.S.A. (playing "A Tour of the West" - filmed from the top of a car (get it?) in full circle vision; I'm not sure if this was open at first or not, one source says yes, another says it was added later)
Monsanto Hall of Chemistry
Space Station X-1 (I never saw this, but it is described as a "satellite view of America as it would appear from space, that is if America was painted by some former animators and set designers.)
In fact, they didn't even start work on tomorrowland until 6 months before opening, and added some prefab buildings at opening time so it wouldn't look empty.
This is why people say that there were either 17 or 18 attractions when it opened - I mean, do you count the castle as an attraction when there wasn't a walk through? Or the frontierland gate? But I listed everything that I could find ever referred to as an attraction (no, I didn't list restaurants or stands) to be as inclusive as possible. Over the next year, Dumbo, a 20K leagues walk through (movie props promoting the filim out that year, the boats in tomorrrowland, keelboats, and casey jr. circus train opened, along with some minor attractions/additional sponserships (aluminium hall of fame, case del fritos, chicken of the sea pirate ship, maxwell house house of coffee, etc.).
Anyway, I think the point is pretty clear.