^Don't know why they couldn't grab Lord of the Rings for the parks, but with Potter, there were supposedly issues with Rowling and her "iron grip". Rightfully so, I'm sure.
But I think Fitzgerald was in charge of WDI at the time, so I don't put it pass Disney that they screwed up either way.
As to the original poster, Disneyland was the first park so it got the nick name "The Magic Kingdom" or "Walt Disney's Magic Kingdom" first. Its tagline, or whatever you want to call it, is truly "the happiest place on earth" (though it's "Where Dreams Come True" right now, doubt that will last past Rasulo).
Technically, the park in Florida is called "Magic Kingdom" and don't think there is supposed to be a 'the' in front, sort of unlike Disneyland's moniker.
In the end, I don't think there is really any official way to look at it though, as it seems everyone comes up with something different.
The way I see it, Disneyland in CA is the original, "The Magic Kingdom" or more appropriately "Walt Disney's Magic Kingdom".
The park in Florida is "Magic Kingdom" with no 'the'. When it comes to Walt Disney World's overall tagline, I think "The Most Magical Place on Earth" is fitting, and kind of fits along the lines of the DL one. Similar but different. Some prefer "The Vacation Kingdom of the World". I guess that is ok too. Don't like it as much though.
What they need to do is stick with one thing and stop changing it every damn marketing campaign.
ok, I'm done...don't even know why I wrote this whole thing. haha
As for Park names, yes, it matters...but I am far more concerned with the actual future creative direction and substance of the park rather than the name. But hey, we're a superficial society so...