I don't actually like having to type "YMMV" over and over again. It actually ought to be assumed. However, I have recently read a dumb-ass discussion of what the term means, so let's take a quick look, shall we?
YMMV means "Your Mileage May Vary", a term originating in automobile commercials because people were concerned about gas prices, and the fuel efficiency of a purchased vehicle was of importance. On the other hand, actual mileage varies on the basis of road conditions and the maintenance of your vehicle. No one can guarantee you what your mileage will be outside of optimum conditions, and, if you live in a litiginous society, you might want to make sure that you are not falsely advertising your product.
Did this get used to lie about fuel efficiency? I'm sure it did. Does it get used by trolls today on the InterWebs? I'm sure it does. But neither of these facts take away from what it's meant to do: Avoid bogging the auto manufacturer in lawsuits about small variations of gas mileage in the real world, and avoiding bogging the speaker down in minutia about each person's individual experience and/or nitpicking in the InterWebs.
It says, "The point I am making is valid whether or not your experience differs from mine." It also says, "I am well aware that your experience is different than mine, and I am not trying to claim that I know the One True Way." Because, as with the auto manufacturers knowing that they will be hit by lawsuits for minor differences in mileage, we know that all kinds of interesting conversations on the InterWebs get shut down over cries of "One True Wayism" and loops of logic where, given a proposition "If A, then B, and if not A, then not B", the only real responses are "Well, what if A?" "Well, what if not A?" and "Well, then how come B sometimes doesn't occur when you don't have A?"
We have all seen it. We all know it happens. And, just as the YMMV in the auto commercial doesn't prevent all frivolous lawsuits, neither does YMMV on the InterWebs prevent all frivolous responses. That's just too much to hope for from human beings.
Sometimes, true, YMMV might mean "Fuck your experience. I have MINE."....but to imagine that this is all it means is a little naive. In this case, it surely also means "I think you're a fucking idiot. I think you're incapable of producing a meaningful argument. I think you're living in a bubble of self-love that deludes you. I don't think you matter."
See, here's the thing. You take take a game as seriously as you like. Some people are playing baseball in the Olympics, or are pros, or are in amateur leagues. Some people are just tossing a ball around in the back yards with their kids. Which is more important? Only a jackass would think they know the answer for all people at all times.
Baseball is not any less respected because some kids play T-ball, or because some mom plays with her kids for fun. It hasn't become any more serious, or any less serious as a result. People who play it seriously, do so. People who play it for fun, do so. Even someone who wouldn't be a mild challenge for a serious chess player might keep a board and play over the course of his entire life, and, you know what? It doesn't affect the game of chess for the serious players one whit.
I've seen some amazing things on the InterWebs. Other GMs, other players, have come up with stuff that makes me salivate. I think, "Damn, I wish I had come up with that" or "I wish I could play in that game"....but other than inspiration and camaraderie, I really don't care what you do in your game. Like anything else in life, you get out what you put into it. It therefore follows that what you put into it should yield what you want to get out of it. And that might not be what I want.
The truth is, YMMV has meaning because it acknowledges a fundamental truth: We do not all want the same performance out of our vehicles, we are not all driving the same model, and we can have real differences in the road surface, maintenance, and weather patterns. If you studied psychology, you will understand that children, not adults, are the ones who fail to realize that other people have different mileage. And it is usually children whose first response is, effectively, "Nah nah nah; I'm not listening". Or, "Just shut the fuck up", if you prefer.
But, like the child who has to learn that he doesn't control the world, the poster discovers that the world doesn't simply go away, and the InterWebs are still full of contrary opinions. Eventually, either the child grows up or he does not. Worrying about people who want something different than you, unless it is in some way harmful, is a waste of time. And I have yet to see a cogent argument about how the casual gamer hurts the game.
The guy who demands that The. Game. Must. Be. Serious. is just the flip side of the guy who says "It's all just playing elves in your parent's basement, anyway." Both ignore the wide, wide spaces between, in which most of us play.
Of course, YMMV.