I would have thought the opposite - they're just MacGuffins. It's elements like the frictionless corridor, the platforms room, the inverted ziggurat, etc that make it WPM, isn't it?
But does this mean it's not GH, or FR, or Eberron, unless it follows a script? And what if WotC publishes something tomorrow that changes the script?
I don't think a setting is a script. It's a bundle of names, places, tropes, events - more-or-less canonical, more-or-less paradigmatic.
Sure, but what you're describing is something tantamount to a lie.
Whereas if someone on these boards says "I'm running FR, but I ignore Elminster, Drizzt and the other over-powerful NPCs" it's pretty clear what they're talking about.
I'm sure there's someone out there who thinks that the Suel origins of the vikings is essential to GH being GH, but I can't imagine that's a very common view. Compared to, say, the Suel and Baklun having fought magical wars that culminated in an Invoked Devastation and the Rain of Colourless Fire - which is a core conceit that underpins the whole pulp/S&S tone of the setting (fallen civilisations and ancient empires whose ruins are still dot the world).
Well, each person will have their own expectations of WPM, R&J, GH, or whatever acronym we choose.
I think that the real point to all of this is:
Expectations and communication.
If I want to run an Alice-in-Wonderland themed Ravenloft, that's fine. As long as I communicate it. Others may not agree it is Ravenloft, but as long as the players know what they are getting into, then it doesn't really matter.
But when starting up a conversation about an established setting, such as Greyhawk, Ravenloft, Star Wars, or discussing something outside of RPG like Romeo & Juliet, the initial expectation is that you are talking about the same thing.The expectation is based on the published material of whatever sort.
So I put up a sign that we're going to be discussion and reading Romeo & Juliet, it would meet my expectations if during the course of that event we discussed West Side Story, but not if the entire event was about West Side Story.
If the event is noted as a discussion on West Side Story and its relationship to Romeo & Juliet, that's different.
Better communication, whether it be the title, subtitle, or supporting information, or the explanation that's given after you say, "I'm running an Ravenloft campaign" is what's really important. Because expectations can have a very powerful effect on how somebody perceives what follows.
Personally, I try to avoid having expectations when I go to experience something, because it's really projecting what you think onto the event, instead of experiencing the event as presented. But I'm also smart enough to know that most people will have expectations, and that with an established entity like a campaign setting, those expectations will carry some weight for those people.
Getting back to the original conversation, lore is important in the sense that it helps set expectations. It provides a framework for us to have a conversation, or run a game, without having to spend the time getting everybody up to speed. The more that the event or campaign differentiates itself from "canon," the more time you're likely going to have to address the differences, or at the very least set different expectations.
It's already been pointed out how Lore can have direct and indirect influence on the rules, and that it's also important for the rules to support and not contradict or undermine the lore.
The lore is what turns the game into more than just, well, a game. The lore can be as simple as a description of the dungeon around you and the monsters you meet. That provides the context that informs you that there is an encounter, and if/when a combat occurs, and then engages the combat rules.
MtG has lots of lore, but the lore isn't required to play the game. It can be played with just the powers of the cards themselves, much like a card game uses the numbers and suits. D&D on the other hand is really dependent upon lore to define the game. Sure, if you're using battle mats and miniatures you can just run a combat without any lore, and turn it into a tactical game that doesn't require any lore. But the description of the world around the characters, however minimal, creates a lore for that game.
Essentially, an RPG, at least in this context (video games would be different, for example), is about the actions of the characters within some sort of setting. That setting is defined by lore.