First of all, RDBMS's are not perfect for all jobs. They have serious shortcomings in some areas (semi-structured and unstructured data for example)
A lot of other stuff can be stored in an RDBMS but isn't really optimal for it. Ideally hierarchical directory servers LDAP don't run directly off a relational db.
So there are places for other forms of stores, from BDB to XML, but these cannot and should not replace RDBMS's for most critical tasks.
(Also there is room for real improvement in certain areas of relational constraints in RDBMS's today, but NoSQL moves the wrong direction there.)
A lot of other stuff can be stored in an RDBMS but isn't really optimal for it. Ideally hierarchical directory servers LDAP don't run directly off a relational db.
So there are places for other forms of stores, from BDB to XML, but these cannot and should not replace RDBMS's for most critical tasks.
(Also there is room for real improvement in certain areas of relational constraints in RDBMS's today, but NoSQL moves the wrong direction there.)