1. Must be likable:
The protagonist is the most important figure in your writing. Therefore, if you put forward a protagonist that is not likable, you have just made your story unlikable. And I don't mean we like him a little. We have to find him/her so interesting we want to continue reading more about him/her.
2. Cannot be perfect:
Please don't misunderstand likable with perfect. Your protagonist cannot be perfect. Any character in your story that is, will believable. If your character does not seem real, the reader will stop reading, or read on with prejudice towards your character. Your characters must be believable but not hated. Your protagonist cannot be a forty year old cop falling for a sixteen year old girl. That is protagonist death.
3. Good from the start:
Don't wait to place the protagonist in a good light on chapter three. An agent will decide whether they like him from the first five to thirty pages, or chapter one. If they decide they don't like your character, they will not read on. If you're thinking "I will self publish to avoid problems like that," your making a mistake. If the reader doesn't like your character they will probably read less than an agent, and tell their friends they didn't like it, killing your chances to sell more books.
4. Do not tell show us:
Don’t just say he's handsome, you must show it with a clear or even strange analogy of words as long as you capture the reader. Most people are visual, which is why television is popular. With a book, the words must paint that picture clearly in the reader's mind, increasing their desire to read on. You need to be careful not to give away the ending of your book when describing your protagonist. I also must warn you not to go over the top making your character too good to be true.
5. Do not create an average protagonist:
Even if the character is your average, run of mill, next door neighbor, something about him must be engaging and different. If the reader does not become emotionally attached, he/she may not continue reading, or may decide to dislike the character right then and there. Not only should your character be interesting, but their situation as well. If your protagonist is not interesting your story will not be either.
6. Remove slow dialogue:
You can kill the interest to your protagonist with slow dialogue. Keep in mind that just as you judge people after speaking with them, readers will judge your character after reading their dialogue. If your character drones on with unnecessary conversation he/she will appear boring. Describing the action or situation may prove more beneficial than a dialogue.