Dave Rosenberg (CNET):
What’s important to note is that using a database in a cloud-like manner requires system architects and developers recognize the principles associated with building a massively distributed data store.
Traditional SQL-based databases such as Oracle, Microsoft SQL Server, and IBM DB2 were designed to run on a single physical node/cluster in a single location, typically hooked to unified storage with full control over all software/hardware elements.
Running the databases in a virtualized environment with multiple nodes and very limited control imposes obstacles that are difficult to overcome. The wave of NoSQL databases seen recently is a reaction to these limitations.
Yet another way you can look at NoSQL databases.
Original title and link: Are databases in the cloud really all that different? (NoSQL databases © myNoSQL)