I think it would come in handy to have collections versioned, incrementing (internally) this version number every time the collection is updated.
The need for this arises when you want to have the exact representation of the data somewhere in another place, say, web browser.
If we have a list of most rated movies, and want this list to be always up-to-date in the browser window, we should push data updates to the web browser.
But these updates can arrive out of order, and we must have a way to version not only the updates (diffs) but the fetched data itself (which can't be made using another "versioning" collection since you have no transactions in MongoDB).

What do you think about such a feature?