Figuring out Bestsellers.

Every indie writer has to puzzle about this; why did this book I see on Amazon or in the New York Times  become a bestseller?  I am going to go through Amazon's best selling books, look at five and try and see if I can find a common thread.
The first one is The Great Bedroom War by Laurie Kellogg. It is Book Two of The Return to Redemption Series. Link: http://amzn.to/ZAWNXD  We have a divorced entrepreneur, her charming, but controlling ex-husband, a rebellious 14 year-old daughter with leukemia and her love interest who upsets the hell out of the ex. Sounds like a lively storyline. The book is a Romance Writers of America Golden Heart finalist.
Although intellectuals tend to look down on romances, they are among the best selling books in the country.
The second is A Dead Red Heart by R.P. Dahlke which is also part of a series: The Lalla Bains Series. Link: amzn.to/ZAWNXD  This heroine is an aero ag pilot who drives a vintage red Caddy. When someone leaves a dying man at her feet whose last words are a cryptic message, Ms Bains vies for an opportunity to solve the crime. Who can resist the main character's father who is a Born Again ladies man?
Third up is Homesong by Misha Crews. This book was a finalist for the Bronte Award for Romantic Fiction in 2010. Two childhood sweethearts united under the Caribbean sun amidst old churches and flamingos. Sounds heavenly, doesn't it? However, we have a bitter, territorial grandmother who won't let anyone come between her and her grandson. Many other aspects of the past haunt the small town lovers. Link: http://amzn.to/14OrMPE
Next, we come to In Deep Shitake: A Humorous Romantic Suspense by Patricia Mason. It is also part of a series, but it is book number one. Mo is a private investigator, but mostly she just poses as the 'honey pot' for husbands suspected of cheating. One day she gets an assignment to break into a car and gets stuck in a most vulnerable position. The owner of the car, an out-of-work actor from a popular movie, finds her. Mo and actor Grant Ross somehow get tangled up in a blackmail plot involving the Russian mafia, a stripper, a Ross Grant impersonator, and an insane ex-girlfriend who doesn't accept the breakup. Now, that sounds like fun. Link: http://amzn.to/14OtNet
Lastly, we come to  Sick: A Project Eden Thriller by Brett Battles. Daniel Ash wakes after midnight to the cry of his daughter. Just a bad dream, he thinks. She’s had them before. Yet he can’t help but worry when she cries out again as he pads down the hallway. Stepping through her doorway, he expects to find her sitting up in bed, frightened by a nightmare. Something is burning Ash's daughter alive. Something horrible that is spreading beyond the walls of their home, and taking no prisoners. Compared to Stephen King, Brett Battles weaves a tale about biological terrorism. Link: http://amzn.to/XDG7yK
The common thread I see in these series would be two elements: storyline and characters. From what I see, you need to stand out and do it with humor, suspense,  a new twist in a traditional storyline, and compelling characters.