Hand-waving quite a bit here, but...
It seems like we have a lot of stuff in REST.module and such for getting data *out* of Drupal, but we don't have anything for getting it back *in* to Drupal. But we have a module in core that can already suck in data from external stuff and turn it into native content: aggregator module.
So it seems like what would really round out the Web Services initiative is the ability to take content from Drupal site A and pull it into Drupal site B in this manner.
If there's already a duplicate of this, I apologize.