bold, bright and eye catching. Children choose with their eyes, they are very visual and are immediately interested in something if it 'looks fun and interesting'. So a clear, bold and descriptive cover can quickly entice a reader, especially if your target audience is a child. Your cover should also give a hint, as to what the story is about, there needs to be a connection to the book. I found that if the cover is a illustration taken from within the book, children love to find the 'snap' factor, and the excitement that they have seen this picture already.
It depends on your reader! Writing your book, you must have your reader in mind, who are your targeting. A younger reader, loves rhyme, and flow. Where an older reader enjoys mystery, suspense of what will happen next, and the enjoyment of the plot and build up in a story. There should be a unintentional moral, or lesson learnt. Something that the reader is left to think about and remember. Having this allows for the book to live longer, and encourages further conversations beyond the end of the story.