"It still has a huge lead on RDBMS in a very common and useful workload." There's very little barrier to entry there, though. With 9.4, PostgreSQL will work be

"It still has a huge lead on RDBMS in a very common and useful workload." There's very little barrier to entry there, though. With 9.4, PostgreSQL will work better on a lot of JSON workloads than MongoDB. And postgres will continue filling in features to cover essentially all of the use cases MongoDB offers now. MongoDB will have a much harder time supporting things like joins (and normalization which reduces redundancy), new data types, good (and flexible) transaction semantics, sophisticated indexing strategies, cost-based optimizer, useful constraints, etc. This is one reason why XML databases never went anywhere -- existing SQL database systems just said "we can do that, too" and the XML-only systems were relegated to a small niche. Unless MongoDB really continues to innovate, it will have the same fate. JSON+Sharding is a good combination, but it's just not enough to be a lasting player in my opinion.