This is an excellent question! Here is our Presiding Bishop's answer:
What I find refreshing about this answer is that it summarizes the great features of the Episcopal Church without being shy about our shortcomings--sometimes they are the same things! Part of what we are having to learn in the Episcopal Church is to articulate what is means for us to be Christians in the Episcopal tradition. That is a challenge for many, especially those of us who are "cradle Episcopalians" who have grown up in the church and barely know any other way of being Christian. Yet there are many disaffected and disaffiliated Christians out there and even more people who would not call themselves Christians at all. What does the Christian Gospel as seen through the lens of the Episcopal Church offer them? Answering that question is indeed the challenge.