Plot, Character, Setting. Everything boils down to that. You can weight them so that one or another is more important in a given story, and you can sprinkle in other things, but those are the basic ingredients. I think that character is the key ingredient, especially for romance. If your characters are unbelievable, unsympathetic, or generally unengaging, the story is going to collapse. With really strong character, I can have weak plot or setting and the story still stands - though a really good story is strong in all three aspects.