I think you have to watch this movie with the mindset of a woman in the 1940s. Otherwise, it's a little creepy, but not truly frightening.
The Rebecca of the title is never seen in the movie, but she pervades her husband's house when the "second Mrs. deWinter" arrives at the manor. It's a ghost story in a more psychological sense of the word, as the late Rebecca's spirit is manifested more by the general attitude of the servants in the household than by anything ethereal.
Dame Judith Anderson as Mrs. Danvers, the housekeeper obsessively devoted to the late Rebecca, is sufficiently nasty and devious, but as a woman in the early 21st century, I'd have fired her ass rather than put up with her condescending hostility. Looking at it from our time's eyes, I'd wonder about their relationship.
A lot of it comes down to the real main character, the Second Mrs. deWinter, played by Joan Fontaine as a lonely, overly sweet mouse of a woman, pathetically grateful to have the attention, if not really the affection, of a man as "important" as Maxim DeWinter (Lawrence Olivier). Olivier's portrayal is great, by turns romantic and kind and bewildering and tortured. A couple of times, he needed a good smack.
I suppose we can blame some of it on Daphne DuMaurier, the author of the original novel. She never gave the Second Mrs. deWinter a name. That in itself is cause for the low self-esteem the character manifests. It would be easy for her to lose herself in the house, or to be driven away. She manages to stick it out to the end, and you have to give her credit for that.
All that said, this is a movie I come back to time and again.