Two big pluses come to mind:
1) Cost - With Amazon EC2 you only pay for what you use and the prices are hard to beat. Being able to scale up quickly to meet demands and then later scale down and "return" the unneeded capacity is a huge win depending on your needs / use case.
2) Integration with other Amazon web services - this advantage is often overlooked. Having integration with Amazon SimpleDB or Amazon Relational Data Store means that your data can live separate from the computing power that EC2 provides. This is a huge win that sets EC2 apart from others.
