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.
Things to be aware of in the Application
- Instead of one large essay, MIT asks for several short essay questions.
- You should have taken one SAT II Math and one SAT II Science Subject test.
- If you've taken the AMC/AIME, then there is a place on the application to submit your score.
- The application separates out scholastic and non-scholastic achievements, giving you 5 slots for each.