It's nice to see Amazon entering this space as they do have the expertise to keep this up and running as well as provide the scale needed as apps grow.

What i always tho is the ability to run search queries that also involve dynamic grouping (like grouping by random combinations of facets) and providing those aggregated results.

Only thing i've seen that can do this "on the fly" is SenseiDB. CloudSearch/Solr/etc seem to need preprocessing to get this right.