The most useful thing I can do, to start off, is to compare my experience with that of my friends at Yale, whom I visit often and who often visit me.  Compare the reviews on this site of Yale with those of Princeton; the Yale reviewers will tell you that it is possible to skate by, sleeping in and playing Halo, or to load yourself with hard classes.  This sounds about right; my friends at Yale like their seminar classes but stay away from intro classes like they're radioactive.  The Princeton reviewers talk largely about how hard you must work here, no matter what courses you take - but at the same time, the professors in even intro classes take a sincere interest in you.  I've frequently had lunch with one of my intro philosophy professors, and I can't even describe how great that class was.  At Princeton, you don't just read the material and absorb new ideas.  You master the material and contribute your own ideas.  Better yet, the professors and preceptors actively want you to do this.

One of the most striking things I hear from Yale friends is that, at their school, prerequisites for classes and departmental regulations actually matter.  Not so at Princeton - here, the prereqs are largely for show; I've taken two or three classes already without having taken any of the listed prerequisites, and no one bats an eye.  This is a reflection of a general academic philosophy that encourages bright and/or motivated students to try new things and even jump ahead.  Anthony Grafton, a history professor, says much the same in a Daily Princetonian article: “At Princeton, we have rules, but we know how to bend them.” (paraphrase).

Not only that, but the Princeton thesis and JP form a really unique experience - other schools may have senior essay requirements or may call them “theses,” but no one expects as much out of you as Princeton on a senior thesis, and no one has you working with one faculty member for an entire year (or more), much like a graduate student - in addition to regular coursework. 

Socially, all that hard work does make for a “work hard, play hard” atmosphere.  That said, there are many ways to play hard (and just as many to “play soft”).  Start with the eating clubs.  I didn't like eating clubs as a freshman, but now I've changed my mind; they are the ultimate social hubs.  You can use them to party like at any frat, or you can use them to sit down and eat and talk with friends, study in the library, or play video games or pool.  Either way, you'll be making friends.  Not only that, but the variety of characters of the clubs mean that you'll meet more people just trying to find the club that's right for you than you ever would if you just stayed in the residential colleges.  This is what makes the campus feel like one big community - a large machine with each gear a club or college.  This is reflected in the attitude of the students, I think.  People at Yale seem ready to pat themselves on the back and are easily wowed by their classmate's achievements.  But the Princeton community is tight enough and within a small enough community (i.e., the town of Princeton) that it is no longer strange to see your friends doing something amazing.  You begin to react along the lines of, “well, that's just what they do.” After all, they're your friends, and your friends aren't strange to you.  That, and as other reviewers have noted, there is a presumption against boasting about your achievements.