This really sounds too enterprisey. It's unclear to me what the benefit is of using SWF, and lost my interest when it started listing business definitions.
"too enterprisey" = a service that large businesses can integrate deeply into their existing applications, Amazon can bill them for, and yet another compelling reason for companies with obscene amounts of money to dip their toes into AWS.
Look at this feature next to the AWS Storage Gateway, recently announced: these are features to get businesses using portions of AWS so that they demonstrate their reliability and cost effectiveness at a small scale. Once a business is comfortable dealing with Amazon then they provide compelling reasons to expand into more of their services (EBS booting + attaching Storage Gateway volumes to EC2 instances for braindead simple cloud compute migrations away from expensive, inflexible on-premises datacenters, SES, SQS, SNS integration with SWF, etc)
Never forget that Amazon is playing the long game.
Look past that. This is super useful - high scale services often have loosely coupled services which pass data from one stage to the other and act on it based on various decision factors. Almost everyone I know invents their own mechanism for gluing these things together - the SWF seems super useful there.
Windows Azure (my former team) actually tried to do this with Azure Workflow Services several years ago, but that got killed around a year into it's existence.
One usecase is analytics workflows. It can also be used to build more elaborate machine learning models (say over data that is too large to fit in one box, and requires distributed coordination)
