After surviving the brutal admissions of 2010, I've learned that the well-rounded student is often the one receiving the rejection letter. (Or, as the trend has been lately, rejection email). Many of my friends were involved in many activities, and did well in these activities without being exceptional. For example, they may be first or second chair viola, played multiple sports, and were active in various other clubs, but were not in the all-state orchestra. Without this higher recognition, good luck getting in.
So what do they want? It seems to me that your best bet is to be what an admissions officer at an Ivy League school once described as "happily lopsided." Instead of seeing you do well in many things, I think universities would rather see you do extremely well in one or two things. So the hypothetical person above would look better if they dropped everything but viola and one sport, committing any extra time to these two activities.
So college admissions officers, if you want us "happily lopsided," then just say so! Stop feeding students this line about being well-rounded.