Being a place where you can live out your fantasies, there are inevitably some disturbing ones. As well as the aforementioned paedophilia, there are reputedly a bunch of people who pretend to have been abused as children (or to be abused children). Plus that "sexy lady" avatar you might fancy...
Haven't there been recent reports that the UK police want a presence in Second Life?
Personally, I haven't got enough hours to do everything I want to do in my first life, let alone having a second - though I can see the appeal to some.