Probably not all that relevant here, but I think that Java's verbosity is a barrier to entry in itself, even if Java programmers do not think so. When I worked in a Java shop a few years ago I would have happily coded up quick extensions and scripts[1] in something like python, but never even considered doing so in Java because there was too much effort involved. I guess I'm arguing that the users must feel that the benefit of writing a script or extension must feel like its much greater than the effort of not doing so and the effort of writing the extension and that Guile is just plain too unfamiliar to most that the effort seems to outweigh the benefits, even if in reality this is not true.

[1] I did actually write a few Python scripts to generate repetitive Java code because I felt that doing so would be much less effort than writing the code by hand.