For what it's worth, my GPA is ~3.8, MCAT 38+, have 4+ years of research experience, multiple first author pubs, multiple international conference presentations, a thesis, ~100 hours of shadowing, a tiny amount of volunteering (<50 hours), and some programming experience. So basically an application with strong research, strong MCAT, and a decent GPA, but with very little in the way of volunteering or other activities. I applied early to 18 schools (range of MSTPs with one non-MSTP thrown in) and got 6 IIs (I assume any school I haven't heard back from at this point is a rejection). Of those 6 IIs, 3 resulted in rejections. Of the remaining three, I should have heard back from two of them about 2 months ago so that isn't encouraging, and I'm not holding my breath for the third one. The fact that I just renewed my apartment lease for 12 months should be a good indication of what I'm expecting to come from this cycle.
There's a perception that MD/PhD programs really only care about research and stats, with maybe only the top schools also putting emphasis on the usual MD stuff. Maybe that used to be true, but I'm not sure it holds anymore. If you look at the acceptances thread for this year, virtually everyone posting an acceptance has a strong MD application in addition to strong research experience. As a supporting anecdote, I was in a group conversation with an adcom member of one program who let slip that the only people she recommended for admission each year were those with music and/or athletics on their CV. Also, many schools I interviewed at seemed to almost care more about non-research activities (one MSTP never even asked me a single question about my research while I was there, I kid you not).
All that said, I wouldn't be much of a scientist if I didn't try to disprove my own position. I had some people who read over my essays tell me I didn't come across as caring a whole lot about medicine (which would have been nice of them to tell me when I asked for feedback before submitting my application), and the same point was brought up at one of my interviews rather bluntly by one interviewer, so that could explain a lot if that truly is how the application came across. But then again, much of the reason I spent more time talking about research in my essays is because that's really the only stuff of substance I had to talk about on my CV (kind of hard to convincingly argue that you have a passion for medicine when all you've done is shadow). I also haven't gotten feedback from programs that rejected me post-interview, so for all I know I was rejected for reasons totally unrelated to a lack of activities/volunteering. Also, Fencer is a program director and he seems to have a different opinion on this matter, so there's that.
tl;dr - My personal experience leads me to believe that your lack of activities, especially volunteering, will be a severe impediment to your success as an applicant no matter what your MCAT score, but my perspective is limited.
