Have you looked at postgresql document store? Amazon put a mongodb API on that and made a service after all
2) Of course. But MongoDB is orders of magnitude faster in some cases, has better horizontal scalability capabilities, better drivers and some nicer semantics e.g. around streaming.
https://hn.algolia.com/?sort=byPopularity&prefix&page=0&date...
No hate, just always amused me that someone on HN had MongoDB as their hobby horse.