I have heard Science Fiction and Fantasy referred to as the fiction of ideas, and I like that definition, but it's the mainstream public that chooses my books for the most part.
The idea led me into the research, which continues to give me more ideas for the story.
From the beginning, when I first got an idea for a story and wondered if I could write it, it has always been the story that has driven me.
I had an idea for a story about a young woman who was living with people who were different, not just superficially different - such as hair colour, or eye colour, or skin colour - but different in some significant way.