MS Dynamics CRM does it to great effect, and I'm hoping one day the Product Team will incorporate it into Sharepoint.
Its the ability to export the configurations of a CRM application and import it into another implementation - minus the data.
So within SharePoint you can save lists, libraries and sites as templates as well as backup site collections, but you can't seem separate the configuration from the data and import into another Sharepoint implementation.
To put it in a real world scenario - think of a staging and a live environment. It would be great to make configuration changes to a list or library in staging and then be able to apply those changes to the live site without loosing the live data. This would be great if it could apply to a single list or library, right up to a site collection. oh, and throw in workflow configurations while your at it, as you can't move them whatsoever.
There may be a 3rd party that does this, I've never actually looked that hard for one, but its a worthwhile feature and much needed by anyone doing deployment.