I love this thread, sitting on a train and reading it since I am looking for something exactly like this to try to make workflow building more intuitive...
if you ever seen how the node editor in Blender works, this is what I would like to achieve for various functions in a workflow engine to work.
But as a learning exercise I would like to build it myself... and I want it on a platform not bound to specific hardware/os so Java/JavaScript seems to be the most natural way to go...
favmarked to look closer at it at a l ater time :)