RDBMS is a great approach. It keeps data consistent, it's good for OLTP (http://en.wikipedia.org/wiki/Online_transaction_processing), it provides access to good grammar, and manipulates data supported by all the popular programming languages. It has been tremendously successful in the last 40 years (the relational data model was proposed in its first incarnation by Codd, E.F. (1970) in his research paper A Relational Model of Data for Large Shared Data Banks). However, in early 2000s, big companies such as Google and Amazon, which have a gigantic load on their databases to serve, started to feel bottlenecked with RDBMS, even with helper services such as Memcache on top of them. As a response to this, Google came up with BigTable (http://research.google.com/archive/bigtable.html), and Amazon with Dynamo (http://www.cs.ucsb.edu/~agrawal/fall2009/dynamo.pdf).

If you have ever used RDBMS for a complicated web application, you must have faced problems such as slow queries...