This *is* an interesting thread. I lived in Portland, Oregon for several years before moving to Brighton, and I was glad that Brighton's more open-minded, artistic, and free-flowing in general, though it's certainly no Portland. Portland, I think, is really embodying the ultra-green and socially progressive movement right now, and I'm a little bummed to be missing it. That said, I think Brighton's pretty decent. It's definitely liberal, and more and more shops and cafes are going green. If you talk to people on the street, they seem to have similar attitudes to people I would meet in Portland. The art scene is alive and vibrant.
But on the other hand... there's a lot of litter. Maybe that sounds like an odd thing to notice, but it's quite striking. Back home, I almost never saw litter. Here (and I don't know if this applies to the rest of the UK), there's just gobs and gobs of it--the grass next to the sidewalk will be covered in candy foils, old soda cans, packs of cigarettes. So while the city may collectively purport environmental awareness, a large majority of folks aren't really backing that up.
One thing I'd like to see here in Brighton are more alternative primary schools, a UK version of magnet and charter schools. I'm disappointed in the state educational system here, and I was surprised to learn that there aren't any state-funded alternatives given how liberal Brighton is supposed to be.
Those are just a couple aspects of "progressive", and folks will have their own preferences. Of course Brighton is all of those things other people mentioned--a huge party town, lots of drunks on the weekends, etc. But the influx of youth, whether for the clubs or the university, are really a driving force behind modern social and political views, so I'll take the bad with the good.