I would second all of the advice from the recent posters. Though I am perhaps a bit nostalgic for what was a wonderful undergraduate experience, I really thing that St. Louis is one of the best places to go to school. Live in U-City - it is extremely cheap and extremely convenient. I shared a huge 3 bedroom 5 minutes from campus (I recommend living West of Delmar - much cheaper and not much further from campus), and my share was $300/month. You do need a car. You might also want a bike - if youre in good shape, you can bike to a lot of cool places from U-City, and the beautiful architecture in the CWE and surrounding areas is fun to bike through. I never found safety to be an issue (although a cop was murdered point blank at the intersection of Delmar and Midland last year...) - our apartment was never broken in to and no one I know was ever mugged, despite often leaving our door unlocked and walking home at 4 in the morning. WashU and U-City cops patrol frequently (especially along Kingsland Avenue - there is a police station a few blocks West of Delmar on this street).
Re looking for a 'liberal, green' community - St. Louis has its fair share of this, you just need to seek it out. There are great places to eat, hang out, and shop in South City and Soulard. There is really good 'ethnic' food - good Asian and Mexican grocery stores and restaurants, a couple of good cheap Ethiopian restaurants, etc - and a fair amount of 'indie' culture.