Some people smoke pot, but if you don't want to then it's completely avoidable and there's very little social pressure. Definitely no more pressure than you would get to drink alcohol at a normal school.
As far as the wealth of the student body goes, Hampshire is a young school and so it does not have a very big endowment. It cannot afford to give away a huge amount of scholarship money. However, there many people there are on partial scholarships and a few are there on full scholarships. More importantly, there is an elitist ethos there such as you would normally associate with rich kids. Even if people are trust fund kids, you would never know because almost no one wears expensive clothing and quite a few people run around wearing clothing they could have gotten in middle school. There is a very liberal attitude on campus and so you never hear people complaining that if poor people would just work harder they'd be fine. Everyone makes great efforts to be PC.
To the charge of artsy-fartsiness, I can only say that Hampshire does attract very creative people, but it also tends to weed out people who aren't serious and diligent about their art. The professors will not tolerate perpetual flakes or people who do not consider their craft thoughtfully. That said, the great thing about Hampshire is that it's easy to have your studies and participate in dance, music, etc. extracurriculars, sometimes quite seriously. They don't make you choose between your interests; you're encouraged to incorporate all of them into your college experience. Also, there are plenty of people there who major in the sciences and never go near a dance show.
Hampshire does attract more than its fair share of anarchists, but they are still a small percentage of the student pop, and certainly make poli-sci discussions interesting.
Idealism runs rampant on campus, but its oddly wedded to a studied cynicism. Let's change the world, but isn't that so typical. The student body tends to believe that big changes are needed, but students vary, to all extremes, in their confidence in possibility of making a difference.
As to the charge of slackerdom, I refer you to my artsy-fartsy answer. Hampshire has a very open door policy and admits many people do not necessarily look good on paper (people who did not take the SATs, or graduate from high school). Often times, these people end up being great students, but this wide net policy means that the school winnows the slackers from the freshman class pretty aggressively. Not that the school actively kicks people out all the time. A lot of the time people realize that they really are going to have to do work and leave. Don't think that our lack of grades means we get a free pass.