MIT Admissions wants to view their incoming freshmen class as a team, so that together they represent a broad and diverse variety of skills and interests. In this light, it doesn't makes sense to admit a thousand well-rounded people, but rather a pretty diverse group of individuals - hence its better to be "well-pointy" than "well-rounded." Focus on your interests and really excel and devote time to them. Don't try to be good at everything. Be great in a few things. To be clear though, it's a given that an acute interest in and aptitude for STEM fields is a huge plus for MIT.
MIT Admissions Blog wrote about the Selection Process, if you're curious to know in more detail the exact process by which applications are judged.