This article is fairly well-balanced, but the hype around these "hashes in the sky" is completely out of control. This article tries to be objective, but it's not doing enough to counter the rabid zeal that is gripping silver-bullet-seeking, Twitter-wannabe young web developers. My observations are that too many people are making the following mistakes:

* Overestimating their need to scale - a powerful box with a cache-backed site can serve a LOT of hits. The vast majority of apps will never need to scale beyond this, don't kid yourself.

* Underestimating the amount of work and the cost of failure in maintaining data integrity at the application level - for all but the most simplistic applications you will end up writing a ton of code that replicates what a RDBMS does, except slower, with more bugs, and less generality.

* Underestimating the importance of data integrity - When your massive database with billions of records has subtle data integrity issues, how hard will it be to fix? My guess is that some truly nasty situations will arise and over time the hype will be tempered by the horror stories.

* Underestimating the constraints this puts on future development - Sure greenfield apps may seem like a great candidate for a document-oriented DB, but how often do apps look like the original version 1 year later, 2 years later, 5 years later. With a relational database your bets are hedged automatically. You can go in a million different directions with your data. The relational model gives you orders of magnitude more flexibility than complex algorithms designed for map-reduce-style scalability. If you don't need the scalability you are throwing an awful lot of flexibility out the window for nothing.

* Underestimating how relational their data really is - It's not just that developers understand the relational model, it's the fact that it's a rigorous theoretical model that actually models relationships in an academically complete way. Sure, real life RDBMSs don't live up to the theory, and sometimes things are too slow to be practical, in which case you fudge things as necessary or add layers of caching. But at the end of the day, the relational model can cover pretty much anything, where these hash-databases give you a small set of scalable functionality and some clever algorithms to accomplish a bunch of different things. The things you can accomplish are not comprehensive in a theoretical sense, they are just techniques that have proven useful for a number of applications recently. However the limitations are not as well defined as a RDBMS.