Hi Kevin,
I think credit based tokens are still a grey area now. This is because it will be difficult to determine how these credit contracts will be resolved when default occurs.
I think what SALT lending resolves this problem by holding your cryptocurrency as collateral and gives you fiat for that cryptocurrency which I think is really clever.
I think there is a limitations for credit tokens protocols for now, as enforcing these contracts and enacting punishment on parties that do not honor the contract is really difficult.
I've gotten hold of some SALT as well, can't wait to use it! lol. A generalised credit protocol however, might prove to be pretty big. So far SALT's focus on crypto collateral is much easier to pull off though.