I don't think 0% or negative interest loans would happen simply because you then take on both the risk that bitcoins could fall in value and that the borrower will default.  However, loans will happen and people might borrow in terms of some index based on real value (a basket of tangible goods of some sort).  They'll take the loan in bitcoins and they'll repay in bitcoins, but the repayment amount will be calculated based on this index rather than be a fixed percentage of bitcoins.  This affords the lender diversification out of bitcoins with a fixed, real return and the borrower is not taking on the risk that bitcoin has a huge appreciation in value.  In short, this loan structure would eliminate the risk inherit in bitcoin volatility from both the lender and the borrower, it offers the lender diversification, and it offers the borrower the funds they may need for whatever venture they might be contemplating.