Unlike most commenters here, I have been to Burning Man many times and found SL through its regional event here. I see a lot of the aspects of the Burning Man ethos here, really they are human aspects the 10 principles were distilled from. No need to go over them, but I would just mention for one, the gifting is immense here in SL. Most sims are gifted to enter, with no cover charge. This is just like the theme camps at Burning Man. A total rein of creativity and interaction in both environments. To me, Burning Man is the same as SL without having to shlep your ass out to such a massive inconvenience of extreme camping, but that aspect of the RL version is what makes it infinitely more experiential, all senses viscerally activated fully.
Here's an interesting interaction on the topic by the founders of Burning Man and Second Life...