I've always found religious prohibitions on "usury" to be particularly disturbing. My parents are from Lebanon and are familiar with people who've gone through life with only cash transactions. The tight-knit communities there make things easier for people who won't pay interest, but even so, it must cool growth. Saving up to buy a car or a house in cash is no small order, and forget about entrepreneurship. (I wonder if applying to YC would be verboten...)