Intense, driven student body. Not competitive--I think that's a myth re-told by lower ranked schools.
If you really enjoy studying and book-learning, this is the school for you. Their clinical experiences do not look as strong as similarly ranked schools. Out-of-state students get a pretty sweet deal. Also, the school projects a very research, academic-oriented image, but not many MD students are involved in research. In a few words, I would describe Southwestern as intense and vividly trying to cultivate a sense of prestige about the school. A very good school for young, smart people who just KNOW they want to be doctors.