Bitcoin, is a real currency that can be used to store and transfer value. The legality of it around the world is being set now. With the Senate hearing this week the US is learning where the US Government sits. If you were looking for a detailed review of Bitcoin, I 'm sorry. The main difference is that now one creates and distributes Bitcoin. New ones are found based on an algorithm built into the network, currently its 25 BTC every 10 minutes. Bitcoin have value as a result of supply and demand.
SLL have value because Linden company says they do.
QCoin, I'm not sure. I did find it interesting that your request came up in the top five of the google response. :-)
thanks for your reply, MJC - its clear that bitcoin is qualitatively a different breed from SLL & QCoin. And this is a first for me to come into a google top 5 response - more power to bitcoin