What makes your protagonist interesting? Sparkling blue eyes? Rippling muscles? Brains? Money? Clothes?
Let's reword that. What makes a real flesh-and-blood person interesting? All of the things I mentioned above could be part of what draws your attention in the first place, but they aren't what holds your attention.
Just like a real person, the most important thing about your protagonist is that you have to care about him. I don't mean you have to like him. Many of the great protagonists aren't particularly likable. Ignatius J. Reilly, Holden Caulfield, Jay Gatsby, Scarlett O'Hara, Hamlet, Humbert Humbert and countless others are deeply flawed, sometimes to point of being straight-up unlikable. But the authors make us care about them. There has to be something sympathetic in the way even "bad" characters are portrayed, so we want to stick with them for a few hundred pages.
Some of the things that make a flawed character sympathetic are described below.
A character who is not very active quickly becomes boring. A protagonist needs to protag. The things that happen in the story have to largely be due to her own actions. Maybe she makes the wrong choices, but those choices raise the stakes. We might not like the character's choices, but we want to know how she is going to get out of her predicament, or whether she even will. She can win or lose, but she has to put herself into situations that draw us in, and then through her own actions, get out of them or deepen the peril. When your main character is always a victim and relies heavily on others to solve her problems, she's not likely to be very interesting, or to grow (or fall) during the course of the story.
A clever character who pulls us along with his unusual or profound way of thinking, his humor, and the unique way he looks at the world can make us care about him, even if his actions aren't always (or ever) admirable.
Yeah, OK, your readers might never be expected to slay the dragon, defeat the evil wizard C'na'ard, and make the world safe for the Nine Peoples of Gerkin, but they will care more about your protagonist if he has to face problems they can relate to. Disloyalty, unrequited love, school or work or family that create problems, dealing with a world that is too big to handle, and many other problems can be worked into your story, problems your reader does have to face. If we relate to your character's issues, we care more about spending hours looking at the world through his eyes, watching
Strength of Character
Your character should always take a stand. She should have a goal and do whatever she needs to do to accomplish the goal. The character's journey doesn't need to be a straight line. In fact, it shouldn't be. But it should trend in a general direction defined by her values, whether the reader (or writer) shares the values or not. Her actions don't have to be predictable, but when we get to the end of the story, we should be able to look back and see that the characters actions were consistent with her values.
We have to believe your character can fail. There are so many books, well-reviewed books, that have disappointed me because I never believed the protagonist was in peril. This tends to be a problem in YA fantasy, especially. A "Chosen One" character who is destined to defeat evil is not going to lose, and in some stories, the possibility of failure is never seriously raised. Every dangerous situation is easily defused without any serious peril. The character is perfect for the situation, and, well, let's face it: perfection is not very interesting. While it is unlikely that the protagonist is going to die, failure needs to be around every corner. The odds need to be against him. Death might not be a likely result, but it doesn't hurt if it does seem possible. Failure, however, might be worse than death, and with rising peril and a real likelihood of failure, we can't help but stay interested. It's like the proverbial train wreck we can't stop gawking at.
If your character creates the story through her actions, views the world through somewhat familiar eyes but in a unique and interesting way, is in real danger of failure or worse, and acts in a consistent-but-sometimes-surprising way, we'll be drawn into her world and her life, even if we don't always like her.