I suppose for me one of the things that makes me like this novel is the sense of mystery and the way in which it is kind of like a detective novel. We never really know who Rebecca is until towards the end of the story, and her fate is shrouded in uncertainty and doubt. This is one of the great aspects of this story for me, as we, like the unreliable narrator, have to work out what is going on from our position of not having much knowledge at all about what is going on.
In particular, the figure of Rebecca exerts a curious fascination on both us and the narrator. She is a character who, although we never meet her alive in the novel, very definitely lives on and whose influence reaches beyond the grave. This is what makes this story so compelling and fascinating and gives it its enduring popularity.
I agree with you. Suspense was the feature that prompted me to keep reading and turning the pages. I think the main narrative that prompted this was how the narrator narrates from the future in the first two chapters - of which, first-time readers have no knowledge of what she is referring to. What also makes it interesting, was the highly-metaphorical use of language in the beginning chapters.
Looking back, anyone who wants to understand the novel only needs to re-read the first two chapters, since the themes become clear through the use of metaphors.