I've previously read one of the now classics by Ira Levin - The Stepford Wives. A good short book with a rubbish ending. Rosemary's Baby closely resembles that book in that regard. It's a well written story which is very very bound by it's time and the ending is also utter rubbish.
I came in reading this book expecting something horrific, something deeply disturbing. I've never seen the film and actually had no idea what to expect plot-wise - at all -.
I was sorely disappointed on the horror aspect. I think this is one of the stories where time has moved on. It was probably really scary back then, but today we've become so desensitized by special effects and even more psychotic dealings, and this is mere child's play.
From today's standard I expected them to do something terribly to the baby. I expected gore. I didn't get any. I did however get a mildly creepy book where just about everybody manipulates Rosemary who is an absolute doormat and never enforces her own wants (which again is very similar to The Stepford Wives where the main character is also a complete doormat (or a woman of her time)).
The one thing that really dropped the rating on this book for me was the ending. It's one of those annoying books that doesn't actually have one. I truly turned the page expecting it to continue on the other side. It didn't. It just ends right in the middle of it and I felt cheated. Boo :/