Hoi,
Stas this sounds absolutely wonderful. Magnus has in his software a lot of use of similar functionality. It is wonderful to see that it is now coming to Wikidata itself. Magnus is moving away from his WDQ and I think that is wonderful on many levels. (It makes me feel like a dinosaur because I still have to learn the rudimentary skills to do what I always do)..
With practical examples like the ones Magnus offers, people really get the idea why this is such a wonderful thing. Only think of every Wikipedia article without a picture :) Or all the items that are part of a specific project..
It will be interesting to see what people come up with.
Thanks,
GerardM