All of the great top-tier horror movies (except Evil Dead II, I guess) have a strong grasp of subtext, taking anxieties from real life and blowing them up along a supernatural or extraordinary angle and preying on them for chills and, ideally, some insight. Rosemary's Baby, as I am the first to point out, is maybe the best of all of them, probably because of the way it stays so close to the surface of reality, only dipping into the depths of shock and horror at key moments, and anchored by Mia Farrow in perhaps the greatest leading role in any horror movie.
That partial adherence to real life might also be why some people don't like Rosemary's Baby - unlike the other famous super-shockers of the late 60s and 70s (a wave that one could argue was kicked into high gear by this movie), it's low on big shocks and instead draws on the slowly escalating anxiety of Mia Farrow's Rosemary, almost all of which could happen to any real-world person with a Narcissistic husband and a difficult pregnancy. Roman Polanski is a master at drawing out the suspense in long pauses, unexpected waits, and awkward silences, but if you go in expecting a Hitchcockian roller-coaster it's possible you'll be disappointed.
It's also not a movie that's built to blow you away all in one go, holding numerous treasures for the inevitable second viewing. I hadn't seen Rosemary's Baby since college, and so many fantastic grace notes stood out to me this time, most of them involving Rosemary's husband, played with smarmy and boneheaded charm by John Cassavettes. The choice he makes is the centerpiece of the movie's first act, and the dinner scene between Rosemary, her husband Guy, and their slightly-too-nice neighbors plays completely differently the second time around. Watch the way that Roman (played by kindly old Sidney Blackmer with a voice like Peter Graves) slowly puts his hooks into Guy, first by making sure he's not too religious to go along with their designs, then by flattering him about his acting career. It's especially chilling once you know what's about to go down.
And a ribbon of beauty-gone-wrong runs through the whole movie, from the pretty pink title cards and almost romantic-comedy-style introduction to Rosemary and Guy, to the score by Krzysztof Komeda, and the inspired casting of loveable old Hollywood character actors in roles both benign and sinister.
Rosemary's Baby is a top-flight Hollywood thriller by a master of the genre, and as such it's loaded with way too many great details and effects for me to go into without spoiling the whole thing (also, it's kind of late and I'm sleepy. Hail Satan.