Hey Helen,
I see the specific affordances in SL as being multi-faceted. SL has some great challenges, but also provides a great deal that Virtual Worlds provide in general.
Specifically, VWs provide spaces to develop difficult or impossible scenarios to experience, from the infantecimal to the gigantic. A user can become tiny in perspective and see the structures of a cell or become giant and stand next to a 3D working model of our solar system (I have one, it rocks!) Not only that, but the social presence of the avatar makes the learning environment truly flat, where people can't judge based upon mere outward appearances, but rather on actions and ideas.
Much more where that came from and in fact have the opportunity to get some builds developed with the company that I consult for, Firesabre Consulting.
Thanks,
Jeremy (Jeremy Braver the Fox - SL)