* you don't need transactions
* you don't need foreign key constraints
* you don't need ACID-compliance
At least the NoSQL folks tend to be honest about their products not replacing the traditional RDBMS approach....Granted many of these don't apply anymore, but then legacies live on interaction-wise. This being said, I know very few real Pg users who harp on MySQL anymore. Most of us figure that Oracle does better advocacy on this area than we can.