The speed at which you can develop is unparalleled in my experience as a full-stack web developer. Convention over configuration is heavily used within Grails and once you start to understand these 'rules' allow rapid development. I like the system they have in place to allow great separation of concerns (Views, Controllers, Services, client-side assets, config files, etc.). The groovy language is a joy to work with compared to traditional Java - some of the built-in methods feel very LINQ-like if you are familiar with C#. These really goes hand-in-hand with Grails GORM database wrapper that allows for terse querying/persistence. A huge plus is that changes to Class files can often be reloaded at runtime which makes development much faster than having to re-compile afterwards (this isn't perfect though). Spring is a huge part of Grails and has been very nicely integrated.

Super fast configuration for API, a lot of plugins

It is very easy to start a Grails application and start a web project. If you are unfamiliar with groovy, it has a fairly low learning curve if you have some experience with Java. Grails uses a programming by configuration system, where things must be named a certain way to get the magical grails metaclass interfaces. To do more advanced configuration, the maintainer should be familiar with Spring, as Grails uses Spring beans under its hood.