For me, it is a combination of believably (do the characters belong together?), realism (do I believe the characters would do that?) and likability (do I like the characters in this story? I don't have to like everything about them, but in general, I have to find something to make me root for them.). If the story makes sense and the characters are acting in a way that moves both the story and the romance along, and makes sense within the confines of the plot, then I will keep reading. I don't need on the page sex to see that the romance is growing, but I am not against it either as long as it is tasteful.