I have to say that Donald was never one of my favorite characters. I found him hard to understand and his temper made him hard to like. He was mean and angry and I was fairly certain from a young age that he was saying bad words. Interestingly, the reasons I disliked him are the reasons that so many people find him engaging.
Donald crosses the line between protagonist and antagonist. When things go badly for him, he does not calmly look for a solution, he yells, stomps his feet, and carries on until either, he gets his way or he is defeated. He is, in a way, the most imperfect of the classic Disney characters. He is also, unsurprisingly, the most forgiven. No matter how often Donald loses his temper and yells, his friends forgive him, love him, and give him another chance. We love Donald because he needs grace as much as we do.
As imperfect people who live our lives losing our tempers, lashing out at those around us, and generally making a mess of things; it is comforting to see Donald Duck. He is loved by his friends and family in spite of his imperfections. He is given as many chances as he needs to start over and try to do better. While we may love Mickey Mouse for his optimism, we know in our hearts that we probably have much more in common with Donald. We are deeply flawed people who rely on the power of grace and forgiveness to get us through from day to day. Donald Duck gives us hope that no matter how imperfect we may be, tomorrow is our chance to be better.