Two contrasting definitions under "Brown University" at Urbandictionary.com reveal the dichotomy: "These students are often tree-hugging, liberal hippies who take their classes pass/fail," versus " Much to the chagrin of the public, Brown students do not typically misuse the Open Curriculum and are not tree-hugging, pot smoking hippies."
To be honest, Brown is almost "officially" liberal. Here, Republicans are heavily criticized and wooed at every occasion. Once I saw an inflated plastic Bush doll hung with a +++ down someone's window.
This goes hand in hand with the school's liberal approach towards shaping one's own education - thus the "no curriculum" and "take as many classes pass/fail as you wish." However, most students take very few classes pass/fail. The University encourages students to try out different courses in fields that are outside their concentration and comfort zone. An Economics student may choose to take a poetry class pass/fail if s/he wants to not worry about dropping his/her GPA. Keep in mind that Brown is a very selective school. Students here, even though they don't seem like it, care a lot about their GPAs and most students put as much work into classes they're taking pass/fail. One of my Art History professors once noted that he was always surprised when he learned which students in his class were taking the class pass/fail, because he never noticed a difference. Most students take courses within their concentration for a grade. Personally, I have never taken a class pass/fail, except for some writing classes that are mandatory pass/fail. At the end of the day, we all know that Brown will end one day and we will have to get a job with our transcripts. And employers can see which classes you have taken for a grade and which ones pass/fail pretty easily. We are not going to take all out classes pass fail unless we have a pretty fat trust fund waiting for us - which some of us do have.
Let's move on to some other aspects of the stereotype - pot smoking. Yes, we do smoke quite a bit. On 4/20, the offical "pot smoking day," students all go to the Main Green at 4:20PM and smoke up on the grass. Pot is a huge part of socialization, but that is not to say everyone smokes up. I'm sure there are certain social groups who socialize in other ways, but it's pretty common.
About being "lazy" : I always say Brown gives you the freedom to get what you want out of your education. You can get straight-A's by choosing the easiest classes and doing only what's necessary. Some people obviously do this, but some people honestly do work their asses off. It all depends on your own personal goals. However, when compared with some schools such as Harvard, the academic environment at Brown is way more relaxed. There is not much competition and people are willing to help each other.