HSTORE still is only storing key, value pairs. Nested HSTORE is something we might get in 9.4. JSON/V8 does allow nesting but doesn't allow indexes for arbitrary value access - something that MongoDB easily does.

So depending on your use-case and the type of documents and queries you are dealing with, neither the current HSTORE implementation nor the current JSON implementation will work for you, while MongoDB's model might.

But the reverse is true too: Some use-cases are highly relational and it would be really painful to do that with MongoDB due to the lack of easy joins (you have to do them in your application logic which means a lot more work as your requirements change).

Pick the right tool for the job.

Me personally, I prefer the immense flexibility for querying a relational data set over the flexibility of not having to deal with schemas. The reason is that it's much easier to change or add an SQL query than it is to re-format and re-duplicate all of my data followed by more or less manually implementing as good query plan in application code whenever the requirements change.

But this is a pure matter of taste.

"I mean would PostgreSQL really have had HSTORE if MongoDB never existed?"

Does your back hurt from moving those goalposts?