With arrays and composite type the potential for semi-structured schema was there for decades. The concept just got more mainstream with NoSQL, more easy to maintain and more efficient with indexables type such as JSONB.
Yes hstore is limited but since you can represent numbers and even hstore itself as text, all you really have to do is cast to and from text and then you have a nested structure with almost anything in it...
jsonb is much better obviously but hstore was really nice and kind of under appreciated.
