A little while ago when I was exploring web frameworks - as I tend to do once every few months - I spent a bit of time looking into web frameworks built in Groovy.
Groovy has all the raw materials needed for a powerful web framework implementation. It's dynamic nature and metaclass facility make it perfect for layering syntactic sugar on top of common tasks. It's very easy to dynamically update Groovy-based functionality - very little special framework is required - which makes it potentially very productive. And it already has very powerful facilities for templating, string manipulation, etc. - all capabilities required for web application development.
With all of these advantages Groovy should be a very fertile breeding ground for web frameworks. After all, for all the dozens of Java web frameworks, you could at least expect a few out there harnessing the power and convenience of Groovy.
Sadly, this does not seem to be the case. You can use Groovy to embellish plenty of existing Java web frameworks, but precious few web frameworks appear to have been built from the ground up with Groovy's power features in mind.
I'd happily be proved wrong on this point, but from my searches Grails seems to be the only real show in town.
I like Grails' web framework. It is an excellent showcase for the power of Groovy. What I don't want is the full stack Grails experience. If I were to use Grails as a web framework, I would want to embed it in an Impala-based application with a modular back-end. It was originally in the Grails 1.1 roadmap to separate the web part of Grails. However, during a recent meetup, I got it from the horse's mouth that this wasn't going to be happening any time very soon.
According to Graeme Rocher, he would consider splitting the web framework "if his users demanded it". I think he is missing the point that by not providing a separately embeddable flavour of Grails he is isolating himself from a significant untapped user base - those looking for a capable web framework but not the full Grails stack. Maybe because Grails has become so popular it is a luxury he can afford!
So where does this leave Groovy? I don't think that it is healthy for a language to be too dependent on a single framework for its popularity. The language needs - and deserves - more variety, a choice to suit a wider range of appetites.