The answer is the unique selling point. The work has some combination of story elements that engage the reader far more than its flaws interfere with the reading experience. You just have to make sure your market or target readers like your USP.
For example, take the first book in this critique series, Ascension, Book One of the Water series, by Kara Dalkey. First, it has a gorgeous cover--not something the author can always control. It also has great copyediting--not something the author can always control either. What the author can control is content, and in this case, the pure imagination that goes into this work. When I notice something original and imaginative in fantasy, especially when spinning an ordinary concept like mermaids or Atlantis, I sit up and take notice, because it is not a skill many fantasy authors have, especially not on a consistent basis throughout a work.
What is so original about this work? The author combines not only mermaids with Atlantis, but she adds in underwater aliens. That is, wow, I've never seen that before. Kudos to Dalkey for that. More, Dalkey constantly adds to the original concept, developing it through world-building details that make this underwater environment fit the characters while at the same being different from the world we know. For example, her characters blush blue, some body language is conveyed through gills, there are different "clans" and classes of mermaids--or rather, mermyds--and there are even different types of mermyds (finned versus two-legged). There are many more details than these, ranging from food to habitat to society, but these are some of the most memorable to me. And I suspect the world-building is memorable to many readers.
That imagination and world-building, I believe, is the USP of this work. Coming in second is the characterization, though it was hard for me to admit, especially because it is more flawed. The world-building is flawed too, but I am getting ahead of myself. Concerning the secondary USP, the main character is basically a nonentity. She is an Idealized Everyman--or rather, Everygirl. She thinks like us, talks like us, feels like us, and acts often like us, but she has some traits we wish we had: she is faster, stronger, bolder, and prettier. This technique helps us relate to the main character more, allowing us to imagine ourselves in her place. Oftentimes that will draw in readers more than observing a character going off and doing her own thing, things we know we could never do or would never do.
So those two traits are what, in my opinion, sold this work and paved over the flaws.
The greatest flaw is the flip side of the imagination coin: information handling. In this case, infodumps. In the beginning, I made note of every infodump, but that got overwhelming. So I have to rely just on overall impression, which is that at least a third of this short novel is pure infodump (and worse, some of it repeats). Sometimes a whole chapter feels like it is just about information reveal, not plot, and plot is the major organizing device of a chapter.
Next, characterization. The problem with creating an Idealized Everygirl character is that it can mean a passive character, because you don't want your character taking too many actions your average reader wouldn't or it will ruin the effect you are going for. Because of that, the main character lets events drop and rationalizes things away, and plot suffers for it. When plot suffers so does interest because you begin to wonder what is the point of the work if the main character dismisses the plot concerns from time to time?
Speaking of plot, when the character is active in the story and the infodumps are few, the plot works well, but that alignment does not happen often in this novel. Because infodumps interfere with story flow and because the character sits on the sidelines when she should not, the plot loses much of its tension in key moments.
That being said, the USPs of the novel are greater than its flaws, so the work actually ranks fairly high in readers' opinions. The takeaway is if you are going to interfere with plot this much, make damn sure that the plot itself is inherently interesting (which this one is) and that something else is stellar to pull the reader along (in this case, excellent imagination).
Dalkey, Kara. Ascension. New York: Avon Books, 2002. Print. Water 1.