The movie was borderline revolutionary at the time, but "at the time" is a big part of that equation. A movie like this had rarely been seen back then, but we've seen a ton of them since then, and, as much as it pains me to say it, Rosemary's Baby is miles away from being the best, no matter what nearly every list of the best horror movies ever implies to the contrary.
For starters, it's just not that well-executed. Even if you've never seen or heard of Rosemary's Baby, if you watch it today, you will figure out what's going on about 10 minutes into the movie and it will drive you insane that Rosemary somehow does not. Once that happens, it's a long, slow procession of mundane events, sometimes scary, usually not, until exactly what you think is going to happen does, in fact, happen.
Oh! What about that scene where they finally reveal the baby, though? Right: For one thing, it's hard to even spot. If you've ever heard someone gush about the "baby scene" in that movie, allow me to shatter any perceptions of awesome you may have with this screenshot.
It's a Gremlin, basically.