In other words, if someone asks you "what is the story about?" can you describe it in one sentence? If not, then your story is likely out of focus.
This is a huge problem for me. I write high fantasy, and so to really understand the situation you have to understand a bit about the world. My plots tend to be convoluted, with very strong over-arching themes. "The Queen's Hound", for instance, is essentially a story about loyalty. The MC's loyalties are tugged every which way, and she's constantly caught in the middle. She doesn't decide which way to jump until the very end, and I don't like to give that part away. While this could be called plot, it lacks the actual actions (which is where most of the plot really resides), and it lacks the flavor of the book.
My YA novel is about growing up without letting go of your inner child. In SYW, I was told that that's nice, but you can't really base a book off of it. To have a book, you need plot, not theme. Meh. While the characters do have problems and do end up getting somewhere, and while the book does have a beginning, a middle, and an end, it is essentially a character-driven book.
I think that summarizing in one sentence assumes that your audience can assume a lot about the situation. "Boy loves girl who loves boy's best friend" gives a definite impression, but gives a very different impression if, say, the boy's best friend is a horse and the girl is a shape-shifter. (OK, so I'd never write - or care to read - a story like that, but you get the idea.)
So in response to the main question, I love character-driven stories with lots of sub-plots, but they are very hard to make a good synopsis for, and therefore perhaps harder to market.