Pretty much of the same opinion as Okarin.
I feel it's a valid expression esp. for objective matters - a good example would be some circumstances I'm dealing with - many like to think they know what I'm going through, and how easy/hard it is - based on assumptions alone, etc. But unless you are going through it first-hand, all your doing is talking out of your ass - even if it's with good intentions (bless my parents, they mean well - but they're the sort i'm most tempted to say "you just don't get it". and not in a polite way, heh) Basically, it's a line of thought where it's a clear difference between experience and ignorance. You can never know what it's like to give birth, unless you've actually done so, etc. (odd example, but apt.)
As for matters of taste, well - you can try to explain why you like "X" to someone that doesn't like "X" - and sometimes a middleground can be reached, but it's dependent on why both parties like/dislike "X".
Example - I really cannot stomach rape or torture in VNs (so things like Maggotbaits, Euphoria, etc) - primarily because I'm close to someone that suffered a dark and grisly hell earlier in their life. So, I just cannot fathom taking any kind of pleasure from reading portrayals of that kind of suffering - even though it's obviously "not real".
But, that being said - it's purely based on my bond with the one involved. Someone that doesn't know the person - and cannot associate with such a dark subject in terms of "real life" will more easily just take it for the story it is. (take my enjoyment of standing off against the cops in the GTA series - the carnage is cathartic - but if I had been a victim/or knew a victim of some massive shooting in recent history, I suspect I'd be harder pressed to find enjoyment in it. Ultimately taste is a subjective matter.
Anyway, that's my thoughts on "You just don't get it". Note: I volunteered those examples, but I would be grateful if no one derails the thread by focusing on the exact content in the examples themselves. Instead, stay on the topic of objectivity vs subjectivity. Thanks