Here is one person's perspective...
I want to know that people have a life outside of medicine. It makes them better balanced people which makes them better doctors, and also better able to survive the rigors of a residency. That life can take the form of extracurriculars like lilycat mentioned, or it can take the form of sports or hobbies. It doesn't matter what it is and oh by the way, those resume padders (things you write but didn't really care enough about to participate in fully), well, they are just that, padding. Being able to talk passionately about your activities (or even just one activity) is a definite plus. Anyone who can't tell me what they do in their spare time (even if it is just to talk about their research) is a red flag in my mind and yes, spending time with your friends/ signficant other is an activity too.
Having said that: Great extracurricular activities will not outweigh poor academics. EC's are a plus, they are not a panacea.