Recommend Paper "The NoSQL movement -How to think about choosing a database."
Today, I like to recommend you read this paper:
The NoSQL movement, How to think about choosing a database.
You may wonder why NoSQL and BigData are so hot, and whether RDBMS is not relevant, all your questions will be answered in the paper. In essentially, the requirement of new generation app requires NoSQL databases, and such business need is the main driver, but not technique requirement.