Jon Udell posted some more comments
The most exciting thing for me about E4X (and variants)
is that it provides a path away from the morass of "passive" XML
configuration files that currently plague web development. My
students (hand-picked, all with good GPAs, most with industrial
experience) have had a hard time getting the "standard" Java
web toolset---Eclipse, Ant, JUnit, Hibernate, Tapestry, and Tomcat---up
and running. Most of their grief has stemmed from the fact that
much of what developers care about is in XML config files, for which
there isn't any kind of break-and-inspect debugger.
other "pure scripting" solutions are easier to use in part because
all of the config glue is in the language the programmer is using,
so she can bring all her programming power tools to bear when something goes
wrong. (Oh, and she doesn't have to wedge conditionals, loops, and
the like into some hashed-together XML syntax.) From what I've seen
a way to migrate from the existing undebuggable mess toward programmatic configuration.