The onus is on flexibility: Second Life's developers, Linden Labs, want every character in their online world to be unique and the character creation tool offers a vast number of customisation options to that end, from how hairy the back of your neck is, to how bulbous your eyeballs are.
Creating a character is a skill in itself, and whenever a minor dignitary visits there are competitions to see who can create the most realistic-looking avatar for them to use.
Such visits are fairly frequent: Cory Doctorow, SF author, launched his new novel inside Second Life recently, releasing it as an in-game object.
Once you have a body to walk around in, things get a little vague, and a bit slow to load. The world loads iteratively, starting out simplistic and low-res, and gradually gaining detail the longer you spend in one spot.