Please correct me, if I'm wrong. But in my understanding the Steem Dollar is backed by the full Steem network. Only 5 % off all Steem that gets created by the Steem blockchain is issued in Steem Dollars. Now the share is much smaller. Dan had written a good post about Steem Dollar stability enhancements. This kind of stable digital currency is really new. It seems to me, that the concept of such a digital currency will evolve over time. And as the Steemit Whitepaper stats it: this is a big experiment.
The Steem Dollar distributes also 10 % interest per year for the holders.