> Now, I know from experience that there are scenarios where RDBMS's don't work very well, but - sorry to be harsh here - I don't see too many people making those 'RDBMS/ACID/whatever sucks' arguments here have actual experience with that kind of workloads. Also note that most shops using fancy KV stores and such *also* use relational databases for parts of their data. Its simply a question of the right tool for the right job.
One thing I see from people who do a lot of work with Postgres and understand it very well is that there is a lot of flexibility on the data structures used and methods to access those data structures that have been optimized by developers competent in the math over the course of decades. So whatever algorithms are used to drive these NoSQL databases and make them fast are probably already implemented as good or better in Postgres, but it does take a lot of familiarity with how the data is stored to make use of this fact. You also have the ability to make plugins to add new data types and storage methods for whatever use case you need.
I guess to distill the point is that many of the techniques which make NoSQL databases fast can also be used with SQL databases if you know how, and you get the benefit of the decades of research and development that has gone into the traditional SQL engines.