This is strictly as case of critic-speak. It is meant to convey several possible scenarios. It may mean that the movie at hand might offend you -- or shock you or anger you or leave you scratching your head wondering just exactly what it was that you just watched. It's kind of the one-size-fits-all warning, covering everything from the tight-assed to the intellectually-afflicted without singling anyone out. At bottom, what it means is that the movie in question isn't a mainstream offering. But honestly the "not for everyone" that might festoon Hobo with a Shotgun and the one that applies to, say, Synecdoche, New York are very different.
From a critic's perspective, it's a useful phrase. It just doesn't tell the whole story. Theoretically, you should be able to gather why from the full review. This, of course, presupposes people will read the full review. Since most of us are called upon to provide star ratings -- or some equivalent -- for those who aren't going to read the review, that may not be happening. I confess that I've never understood how people who are too busy to invest the time it takes to peruse 500 to 1100 words can possibly have the leisure time to undertake two hours of movie.
Of course, the idea is simply to minimize the number of people who get their withers wrung because we've recommended something they hate. It works better in theory than in practice. First of all, there's the simple fact that a movie you might consider completely innocuous and inoffensive is still going to offend someone. Anyone remember a little picture called The Water Horse (2008)? I wrote, "It contains nothing that I can imagine would offend even those with the tenderest sensibilities or the staunchest religious scruples." So what happens? I get an angry letter from someone because I didn't warn her that it contains a near-drowning incident, resulting in undoing "years" of work to get her grandchild over a fear of water. So it also is not for everyone. (We won't even discuss the notorious episodes of me corrupting innocent Hendersonvillians by recommending Bad Education (2004) and I Heart Huckabees (2004).)
There are other instances of critic-speak that might be worth considering, though the ones that occur to me are generally more easily defined than "not for everyone." Take for example "emotional resonance." That sounds pretty deep-dish and even highfalutin, doesn't it? Well, it isn't. It means nothing more nor less than the movie made the critic cry -- or at the very least tear-up. It simply sounds classier and more intellectual. Regular folks may cry like babies when Bette Davis hands in her dinner pail in Dark Victory (1939), but the critic admires the film's emotional resonance. Do I use it? Of course, I do. Don't be so damned silly. Do you think I want to sound like some kind of weepy pushover?
Far more dangerous is "deliberately paced" and its close relatives "leisurely paced" and "beautifully restrained." "Deliberately paced" -- though it may suggest that it's the opposite of "accidentally paced" -- is a way of saying that a movie is slow without saying it's slow, since slow sounds like a bad thing. The critic means you to understand that the motion picture you're about to see is not action-packed, but that it has other, different merits. You might, after all, consider seeing something that's "deliberately paced," but shy away from something labelled "slow moving." The truth is that it's slow moving in either case, but in one case the critic is trying to sell you on it anyway. This is all pretty subjective, of course, and there's probably a 50-50 chance that you're going to be bored. Still, Ingmar Bergman's Saraband (2003) is pretty darn deliberately paced and it didn't bore me in the least. Michael Bay's Transformers: Dark of the Moon is certainly action-packed and it bored me stiff.
"Leisurely paced" is pretty much the same thing only not as slow -- like the difference between a gravely debilitated, very old snail and a spry tortoise. Oddly, however, this is rarely used in any sort of positive sense. Instead, it tends to convey the sense that the film is not very exciting. The phrase "leisurely paced thriller" -- which I've seen used more than twice -- is hardly suggestive of a succesful blend. It is, however, a shade preferable, I suppose, to "tepid thriller." At the same time, I've never seen "leisurely paced" used as a positive quality.
"Beautifully restrained" in my experience almost always means I'm going to wish I was just about anywhere else doing anything else. It is almost exclusively applied to movies that critics generally adore and most everyone else falls asleep during. Not only is nothing much going to happen for most of the film's running time, but the conclusion isn't likely to be appreciably more lively. If the film is in a language other than English, is called "realistic" and/or "minimalist," and is abnormally long, look out!
My favorite example of this was a recent encounter, Chantal Akerman's Jeanne Dielman, 23 Quai du Commerce, 1080 Bruxelles (1975). What we have here is a whopping 201 minutes of Delphine Seyrig cooking and doing housework in real time. She occasionally turns tricks of an afternoon (no, this is no more exciting). My wife wandered in on this at about the half-way mark, watched a little and asked, "Are we going to watch her make this whole meal?" I told her, yes, and from one angle. I added, "I figure she's finally going to berserk and kill someone or commit suicide." I wasn't wrong. I've had better times in dentist chairs. And yet -- you have to look very, very hard to find a critic who doesn't find it profound. I find it profoundly boring. Therein lies the truth of "Not For Everyone."