The most valuable feature is the ability to provision a platform of your choice, such WebLogic, BPM Suite and a lot of other FMW products. It's also easy to deploy a MW platform on the cloud. You don't have to worry about setting up the infrastructure; Oracle manages it. You just have to divert away existing licenses of the technology stack to the deployed platform on JCS.
It's shortened our time to market as we don't have to invest in data centers, computing resources and the underlying infrastructure. The proposition of Java Cloud Service is that, "Hey, here is a hosted platform for you on one of industries best application server i.e. Oracle WebLogic Server". Oracle will provide a platform, the hosting services, and the infrastructure. You just subscribe to a metered or non-metered usage and possibly extend the platform using your existing Oracle's license on the product you are using.
For instance, if you are using BPM Suite, you just purchase a BPM Suite license, build an Oracle BPM platform on top of JCS, and deploy your applications on top of it. Oracle will support the JCS infrastructure and you still get the ability to extend it to suite your needs.
You can scale up and scale down, have a high available infrastructure and even do disaster recovery if you have Java Cloud Services deployed at two different availability zones. It's a very low-cost option for a business to start with. That's one of the best business reasons to move into JCS.