I hate this question. As a non-trad, there are many (many) experiences which have shaped my life. There are several major events, most of them very personal and not ones I particularly want to share with an interview panel. I also hate (hate hate hate hate) talking about myself - one of my weaknesses: I don't toot my own horn very well.
IMO, this question ranks right up there with "tell us about a challenging problem you have overcome". Well, gee, do you want a personal challenge? How about a professional challenge? An emotional challenge? A life challenge? A lifechanging challenge? A family challenge? A physical challenge? Dealing with a crippled family member challenge? Breaking new ground challenge? Give me a little more to work with here. And no, I really don't want to talk about any of them - they have shaped me into the person I am, but they are FAR from being the main thing to make me what I am today. Stupid questions.
Ok. venting done. Back to the original question - I answer with why I went back to school, what jobs I've held, what I am today (wife, mother, educator, tutor and guide for my children, student), and what brought me to apply to med school (and why it took so d*mn long to get there). It only takes about 5-10 sentences. Oh, I also mention my hobbies and what I like to do in my personal time. I don't elaborate much - if they want more information they can ask. Of course, most of this is in my application. Good luck.