Prosper.com is USD-based peer-to-peer lending. You should familiarize yourself with their history, problems, and how they overcame them before attempting a BTC equivalent. Their biggest problem early on was the default rate exceeding the interest rate paid to the lenders. They overcame that problem by doing credit checks and better digging into people's financial histories to give a more accurate picture to potential lenders.
In my opinion, you'll have to either do credit checks on the people asking for loans, or do insanely high interest rates to protect against defaults (which would likely only further aid the number of defaults happening).
To me, one of the most prominent aspects of prosper was that lenders only funded part of the loan. This split the cost of a default across many lenders, and allowed for easier diversification of your funds. Something like that with Bitcoin would be very interesting to me.