How did MIT undergrad experience change me?
I came to MIT as a bright-eyed, clueless 17 year old freshman from Chicagoland and left 6 years later with bachelor's and master's degrees, a network of friends, and a very different world view.
Part of this, I'm sure, is part of any college experience (especially in a mostly urban environment like Cambridge), and is not specific to MIT. These are formative years. You are exposed to more people, people who think differently than you. You learn to live independently and enjoy freedom and take on responsibilities. You learn how to pace yourself. Etc., etc.
When it comes to MIT undergrad life, though, I think the biggest change was learning some humility. At one of the freshman orientations, they ask everyone who was a valedictorian to stand up, and about half the crowd stands. Then salutatorians, and probably 4 out of 5 people are standing. You are no longer the smartest of your peers, probably not even close. You won't automagically understand everything you're presented. You can't really coast through classes, and those who try don't make it past the Pass/Fail first year. That learned humility has carried through to the rest of my life; for instance, I rarely tell people I went to MIT unless they ask, and I rarely try to assert that I'm the best person to do X. There's always someone better.
You learn to push yourself. When you are surrounded by these people who are definitely smarter than you, you rise to the challenge. You skirt the edge of burnout and also realize your limits -- "there's no way I can take as many classes as that person," so stop trying to be the best. I can confidently say that, despite my successes in the work force, I have never ever pushed myself as hard as I did at MIT. Once you've learned to juggle 5 or 6 balls, juggling 3 balls is easy.
You become more liberal (socially and politically). This is probably unavoidable while living at a college in Massachusetts, especially coming from Indiana as I was. But as you're exposed to different perspectives, as you see that morality is not as black and white as you thought it was in high school, as you learn to respect the other students and friends you meet, you inevitably start shifting to the left.
You become more social. In high school, the typical MIT student-to-be is... let's call it "socially undeveloped." And I'd still say about a 1/4 to a 1/3 of MIT students don't get much further than that, depending on how much of their social life they have to devote to studies to keep up. The fraternity and independent living group system goes a long way toward helping their members grow up socially. Even the dorms have their social networks and gatherings to facilitate this. Between living groups and the numerous extracurricular clubs available, you're going to develop socially and be a different person after 4-6 years than you were going in.
Finally, one thing that I discovered which was unexpected was a yearning for the humanities. When your core courses are all math and science, you may find yourself assigning more value to the things that are different. I may never use linear algebra again, but I distinctly remember and have used what I learned from my intro to acting class, my short story writing class, my social studies class on privacy and information, and the music theory and history classes I took for my music minor. That, plus being in a fraternity that encouraged a well-rounded approach to academics, really made me less of a socially-awkward nerd and more of a Renaissance man.