Show which articles are cited in a document
Like "duplicate items" and "unfiled items" there should be an automatic folder that tells you what items are used by the document you are working with, e.g. after citing Rechtschaffen and Kales, 1968 in my thesis document there should be an automatic folder called "thesis.doc" (or whatever the documents name is) which has all the items in it that I already cited. Also there could be a "not in thesis.doc" so you can see what items you put into your library once, but never used anything out of it, maybe you just forgot or wanted to add it later.
Edit: url added above and Simon's message announcing the feature:
In the "document collection" it would be awesome to have "chapter collections" that are generated by the headers in the document (i.e. in word and libre office).
Also, there could be a "number of appearance in document" column, so it would be possible to sort the items like they are in a numbered bibliography.