It's important for developers to have confidence in the tools they use. For years I have been building web applications using Struts (the 1.x branch). The nice thing about Struts is that there are examples for almost everything you need to do. The bad part is that the examples are spread around the internet and the many, many books on Struts (about 3 of which I personally own). Many of the examples are out of date, there is almost no interaction with the developers or the user community, and if something isn't working properly, you basically have to find a work-around. I'm not confident that my issues will be fixed prior to releasing my own project. This isn't a knock at the developers who freely contribute their time, but the platform as a whole was in no way a joy to work with.
Grails has excellent documentation with A User's Guide, Reference Guide, Quickstart guide, tutorials, you name it. With Grails, I can jump on the Nabble forum and have complete confidence that others will help me to find an answer. Graeme Rocher (Grails Project Lead) and other contributors are extremely active on the user group. In addition, I now have complete confidence that bugs will be fixed timely. I submitted a fairly serious bug about not being able to instantiate Domain objects in integration tests. The latest Release Candidate fixed the problem less than two weeks after I submitted it. In truth, it had probably been fixed long before in the nightly builds but two weeks is a dream come true for my framework of choice. The result is a platform that I have enough confidence in to recommend within my organization and for our clients.
This is the way open source ought to be.