trading bitcoins for dollars (and vice versa) on the blockchain, because doing such things off blockchain like Mt Gox is not STRONG...
I agree that bitcoins should be kept on-chain... but this topic is more about other innovative things we could do with the blockchain.
Check my earlier comment:
What about a bitcoin token system applied to high end consumer goods. With this token that the end user receives, he or she can verify that it comes from a particular manufacturer, thus guaranteeing its authenticity.
This is one I've been thinking about.