A lot of buzz about tags lately - rightly so. A new economics is emerging here. The implicit transaction is simple: viewers trade their private information (by tagging content) for public information (by viewing already tagged content). This way, the public info gets more and more accurate - the accuracy of tags grows with the size of the network (viz, classic direct network externalities).
It sounds trivial, but it's hugely important. Every new business model is built on an innovative implicit (or explicit) transaction between consumer and producer. So here we have the first step - a trade of public information for private info, which is fed back into public info.
The next step is the critical one - building a b-model on top of this new transaction. But this first step is significant - it's further than RSS has gotten, for example (because RSS is a one-sided transaction so far).