What makes a book successful?

Reading about the success of 50 Shades Of Grey made think about what makes a book wildly popular. 

I have read a lot of books, that have had  engaging characters, fast paced plots, and fascinating settings.  Yet  they aren't up and fighting on the bestseller lists. So why is they plodding along? I think part of it is that they are missing that  extra”something.” The something that keeps the reader coming back again and again.
Let's take Amanda Hocking.  She is wildly successful, at least  when  she was an indie author ( I have no idea how her print books sell).  I picked up Hollowland awhile ago. Even though I never finished, which was more because of me than the book,  I could see why she was so popular.  Her author's voice had that something.  

SB Sarah points out that some authors are just cracktastic.  I can see that. I have some authors that I love, despite a part of me whispering I shouldn't.  Grace Burrowes are one of them.  I love, love her books.  Despite the fact that her books borders on viginettes. Yet there is something with her books that makes me gobble them up.  ( But I still got huffy when I realised that Lady Sophia's Christmas Wish were released before The Virtuso, but chronologically took place after.) 
Thea Harrison is another. I gobble down her books, despite the fact that the headhopping drives me insane.  I cannot help myself. I have buy her books.

Then there is the authors I love, but who release books with 3-4 years gaps.   Lorna Freeman is one of them. I love her books.  The problem is that since there are so long gaps between books, her earlier books are starting go out of print, and isn't available as e-books.   Even though I am annoyed, I'll still buy the next book.  Wen Spencer is another author. I love, love her books.  It is a mix of fantasy, science fiction with a dash of romance that just works for me.   Still, her last book was out in 2009 I think.   But, just like with Lorna Freeman I'll buy it when it is out in July.


All right. This might have gotten a bit rambling.  Let's see if I can tie it together.
What makes a book  successful is having that ”something” and reader loyality.  The problem is in today's publishing that most authors don't have the time to get the reader loyality, unless their books are successful from the start.  

So, which authors do you want to soar onto the bestseller lists?