As adf8 points out, Penn is large and diverse enough (almost 10,000 undergrads and a similar number of grad/professional students) that just about everyone should be able to find his/her niche there. For example, as adf8 readily acknowledges, her perspective is based on her active social and Greek life, but keep in mind that only 30% of men and 26% of women join fraternities/sororities at Penn, so there will be substantial numbers of students who might have different perspectives.
Also, while in adf8's experience, "[n]ot many students have dreams of going into the Peace Corps and saving the world," at least 27 members of the Class of 2007 of the College of Arts and Sciences took jobs with Teach for America upon graduation, and Teach for America is one of the College's top employers:
http://www.vpul.upenn.edu/careerserv...07cpsurvey.pdf (see bottom of page 3)
Additionally, in terms of the diversity of economic backgrounds of Penn undergrads, about 40% of them receive financial aid. Further, over 500 members of the Class of 2011 (out of 2400 total) are from families with annual incomes of less than $90,000, and almost 300 of them are from families with annual incomes of less than $50,000:
A Look at the Facts, Comparing Penn's Cost
So again, Penn is a very diverse place, and just about everyone should be able to find a comfortable place to fit in.