If you want to know what makes a best seller, you have to understand the workings of a bad one. Writing is about placing your thoughts to paper, making a difference, telling a story, changing the way others think. All this is fine and dandy until you want to sell it. When money is not exchanging hands and the size of the audience is not a concern it is perfectly fine to write whatever you want. Writing can be a great stress relief or a way to remember a thought, dream, or story you heard. The minute you decide you want to have everyone read it or (and lets admit it is a driving factor) get paid for it you cross the line between writing for yourself and writing for others. This time it matters what your audience wants and less about what you want. What do they want? It differs for everyone, but the best way to find out if your book stinks is to have others proof read it; unless you already have a publisher don’t think they will cover your bases.
If your friends, neighbors, acquaintances are counting the pages to the end of the chapter or (heaven forbid!) the end of the book you have failed. Time to either rewrite or toss it entirely. When you find your readers arguing with your narrator you have lost the magic and soon they will be counting the pages. You want your narrator to keep their interest, not drive them away. Be prepared to cry if you find your readers are more in league with your minor characters against your main. Hint- they should be rooting for your main character (pssst- even if he/she is the villain)! Beware the reader that skips pages to get to the good stuff. Steer clear from too complicated. Yes, there is such a thing. Some complication will keep your readers intrigued, while insanely complicated loses their interest.
Stay away from impossible rebirths. Vampires are okay as are werewolves or angels and the like, magic also has its potential, and science experiments carry some possibilities. But all these still have their limitations. Readers need limits or the story runs flat. What good is a main character that only wins because there is no means for failure? Put yourself in the place of your readers. You may think it is an incredible story, but remember all the volumes to which you gave your precious time only to put them aside in disgust. If you want it to sell the opinions of your readers are king and you need to be writing for them as much as for yourself.