Iain M. Banks himself has written up some ideas about what The Culture could or should be in the essay "A Few Notes On The Culture" (1994). This essay can be found at http://www.vavatch.co.uk/books/banks/cultnote.htm . Because the copyright note seem's to allow it, I've copied it into the E2-space. It's also full with background information on The Culture in the more classical sense of that term, like what a culture ship is and how far spread throughout the universe The Culture is thought of. But it also describes well written some of the more philosphical features of The Culture - the features that make it look like paradise or an utopia.