In a romance, spark between the hero and heroine is bottom-line essential. Dialogue, tension, sensuality and characterization are other basic tools. But hark, there is more than the plot line, too. It’s called Texture.
The composition of a novel is much like painting on canvas. The artist first prepares the background of the painting, then the highlights are added. If the finished painting is ALL highlights, nothing stands out. This is true when writing scenes, and we want our hero/heroine to really come to the forefront.
In writing the novel, all of the story elements mentioned above can be one-dimensional, without depth and interest -- if not layered or placed within a setting. Setting could be the description of a physical place -- outside or inside, and weather. A novel's storyline also needs color, such as comparing the green of the heroine’s eyes to a highland meadow, the hero’s gray eyes to the color of steel or ice. Without these textures the story can be perfectly executed, but unexciting/blah, blah, blah.
Without setting in a story, the characters can be no more than "talking heads." The storyline can wind along, lots of action with these talking heads. But without the setting and color, the reader may never enter the story . For example, one perfect setting for a suspense might be an isolated lighthouse, the sound of the ocean crashing against the rocks, a storm brewing in the gray sky, the air moist foretelling rain.
Think of the movie "Jaws." When that particular music occurred, the viewer knew that danger was impending. The writer does not have music, but he can create nuance by using storms, rain slashing the window's glass, sunlight and wind, shadows and pounding heartbeats.
Don't forget the little things, either, like the chip in the rim of a cup, how a person deep in thought may circle it.