I would have to say that the most interesting thing about Second Life is User Generated Content. When I joined they had the phrase "Your World, Your Imagination" displayed prominently on their website. It was this DIY culture that was a big part of what attracted me to the place. While that culture is shifting somewhat, user generated content is still the backbone of the grid. It is the main reason that the grid is so diverse... that vampires, steampunk fans, elves and robots can all exist in the same space, along with countless other subcultures.

User Generated Content in tandem with the convertible virtual currency, the Linden Dollar, has also allowed many people to generate some or all of their income from selling products and services on the grid, including many people with health issues that wouldn't be able to work a more traditional office job.

User Generated Content is also responsible for the large and vibrant art community that has sprung up in second life. Without UGC, things like this particle show put on by Kala Pixie would not be
possible

I think how realistic and the attention to every little detail is the most interesting thing about it. I'm into hip hop and the clothes I see avatars wear there are exactly how people dress in real life to the tee. I mean fitted ball caps with flat brims to coogi pants it's incredible. I also have seen Michael Jordan shoes that look exactly like the real thing.