I know for some people it’s plot, or certain details of the research, or character. I used to think it was character for me, and I still think that helps, but more important is voice.
Until I have a feel for the voice and tone of a story, it’s hard to get beyond the first few pages.
Of course, having said this, I’m sure the next project I work on will decide that no, it’s something else entirely that I need before I can enter the story …