Structure is important.
It's not enough that your story be interesting; it has to make sense and have a definite purpose. Like I said in my last post, if readers don't know where the story is going they're less likely to stick with it to the end.
A lot of book overviews read like a string of tv episodes instead of one stand-alone story. For instance, The Fault in Our Stars starts out with a teenager who is dying of cancer meeting a boy and falling in love. It goes on to talk about a book they both like, the boy's efforts to contact the author of the book, their efforts to meet the author, their disappointment when they meet him, a random scene at the Anne Frank House, and on and on and on.
If you can't sum up your story in one or two sentences, you don't know what your story is about. That's the use of a log line--a short (one- or two-sentence) summary of your story. Into less than fifty words you've got to pack four key pieces of information:
His main problem and/or goal
If you can do that, then you've got a cohesive plot and a story that will work. You've also told your readers what they want to know about your story before they try to decide whether or not to read it.
Here's an example log line for a real book:
A lonely orphan is sent to the Yorkshire mansion of her mysterious hunchbacked uncle. While there she finds the key of a garden that has been locked up for twenty years. (The Secret Garden)
It's got an MC (the orphan), a problem (she's lonely and her uncle is mysterious--any time a mystery is involved its solution becomes a default goal), a setting (Yorkshire; and the genre is obviously something along the lines of children's/gothic/fantasy), and a hook (why has the garden been locked for twenty years? What will happen when the MC opens it?).
That's not a particularly great log line, but it does demonstrate how clean and concise that story's plotline is. Of course there's a lot more involved (including a grumpy gardener, a boy who can tame wild animals, and a strange voice crying in the night), but the main idea of the story is easily summed up and the story sticks fairly well to what's at its centre: the locked-up garden.
So let's break it down and look at how to write one of these for your own story.
We'll start with the main character. I'm personally averse to putting the MC's name in your log line because nobody cares what his name is. Far more important is a few well-chosen words telling what he is. A young boy, for instance, or an old miser. A retired cop. A con-artist. A business guru with a dark past. Tell what makes him particularly relevant to this story.
The main problem should be connected in some way to what your MC is. If he's a washed up actor, it's likely his problem will involve recovering the success he once had. If his problem is to save the galaxy it would make things more interesting to make the MC an unskilled teenager than it would if you made him a jedi knight (or at least start out that way and become a jedi by the end).
Setting and genre often go hand in hand. Both are more important than you might think. If you start a book you thought was about aliens you're likely to be disappointed if it turns out to be about vampires. (That happened to a friend of mine.) Or if you try to read a book about space travel (like I did with C. S. Lewis's space trilogy) and it turns out to be more of a fantasy.
The Hook is the most critical element in your log line. If you don't have a hook in your story then you might not have an interesting story. It's what makes your story different from all other stories and makes people want to read it.
I'd love to go into depth on the subject of the hook, but that would make this post too long so I'll save it for another post. What you really need to know about a hook right now is that it needs to be interesting. Don't describe a mystery by saying it's about "far more than meets the eye." Every mystery is about more than meets the eye--that's what makes it a mystery. Tell specifically what makes this mystery different from all the other mysteries out there. And if it isn't--well, you just might have a serious problem with your story.
That's why I recommend writing your log line even before you start writing your book. It gets what's most important about your book down on paper so you know what to focus on while writing. It also shows up the most serious problems so you can fix them before you've gone through all the hard work of writing those parts. The worst problem I've ever run into when writing a story is not being able to see the big picture--how all the different parts of the story connect to make one smooth plot arc. The log line helps you do this... in just a few sentences.