We wanted to create an engine that is :
We like the idea of the PVM (see http://docs.jboss.com/jbpm/pvm/) and have taken some ideas from this concept while we dropped ideas that reduce the simplicity of GrailsFlow.
Developers including us wanted a workflow engine that has all "basic" functions out of the box, is really easy to understand and extend and is closer to Java/Groovy than to XML.
We have used jBPM in a small installation and OSWorkflow in major installations. Both provide nice and stable engines.
However OSWorkflow does not have a stable workflow definition tool, and not many good example applications. jBPM has most of those tools (eg. a workflow editor based on Eclipse), but not nicely integrated in one application.
Both application rely heavily on XML for the process definition including the related variables and activities.
We have realized large workflows with OSWorkflow, where we had some 5.000 lines of XML for the workflow definition. Also we needed some 80 support classes written in Java on top of OSWorkflow (about 200 classes).
Thanks to Groovy and Grails, we need less classes than the number of support classes to implement GrailsFlow completely with comparable or better functionality.