Scalability
Relational databases are designed around the entities and the relationships between the entities, and need a large investment in hardware and servers in order to provide high scaling. SimpleDB provides a great alternative that is designed around partitioning your data into independent chunks that are stored in a distributed manner and can scale up massively. SimpleDB provides the automatic partitioning and replication of your data, while at the same time guaranteeing fast access and reliability for your data. You can let Amazon scale their platform as needed using their extensive resources, while you enjoy the ability to easily scale up in response to increased demand!
The best feature of SimpleDB scalability is that you only pay for usage, not for the large cluster needed in anticipation of large usage.