The fluff, and nothing but the fluff. One of our mods, cordelia_v defines "fluff" as a romantic story where the main obstacles that stand between the two lovers are primarily in their heads: misunderstandings; prejudices; obstacles set up by third parties, which can be overcome, etc. There can certainly be some angst and dangerous situations along the way, but a happy resolution in the end is also mandatory. Ideally, a bit of cuddling before it's all over, too.