I definitely agree that "well-rounded" is a vague term. But if I were to define well-rounded.... I would have to say it's a combination of academics and personal life experience.
If you are a science major, then you should strive to take courses outside of the science realm; why? not because it will "look good" on your application, but because you will have had a more dynamic educational experience as an undergraduate. If you go on to medical school, you can be assured that you will be a science geek in the end.
That being said....the same goes for your non-academic experience. I believe that you should pursue your interests when the opportunity presents itself. I decided to get a degree in History because I was interested in the subject and knew I was already going to be taking plenty of science classes. I also decided to pursue non-medically related as well as medically related extra curriculars. Of course I have done my share of research, volunteering, and I have even worked in healthcare for several years. But I also seized the opportunity to volunteer at a local elementary school and work with kindergarteners and 1st graders...I also interned for the mayor of los angeles among many other things....
But.....what does this all mean? Do I see myself as well-rounded? Yes, but I also am a little older than the average applicant and I have had my share of free time screwing around to do a lot of those things. ( Especially while I was playing baseball in college). Basically I think that whatever little spare time you have, you should try to be productive and do the things that you truly enjoy. In the end, you can't be anything else but well-rounded.
P.S. forgive me for the long letter, but I didn't have time to write anything more brief. ( a quote from some anonymous historian )