The #1 most important thing is a book cover. If you are considering self-publishing your book then you should continue on for sure, but if you plan to try to publish your book the regular way . . . you can still read on but it's mostly meant for self-publish.
Amanda Hocking was self-published before she sold millions of books and became a millionaire. The book cover that is above is her original cover from when she self-published.
This is Amanda Hocking's currant book cover. Eye-catching isn't it? It's very beautiful :) I love the cover. Anyway, you're book cover should be like this too -eye-catching.
In a bookstore if your cover is a different color (such as white, yellow, red, blue, green) it will catch a reader's eye instantly when they're browsing the store's shelves. The reason why is because usually the book's spine is what is facing a browsing eye, and if someone is walking by and sees an unusual color (for a book cover) it will catch their eye and they will most likely pick up the book and read what it is about. If they like how it sounds, they'll probably buy it or save it for another time (an easily remembered title will help the reader remember the book for the next time they go to the nearest bookstore).
But some people will only remember the cover, even if you have a good title, so make it pop out! :)