Built on top of Spring Boot.
Awesome performance, both in execution and development speed.
Convention over configuration. But it is still easy to access the rich Java libraries.
Comes with GORM, an ORM layer on top of Hibernate. Makes it work a lot more like Active Record with dynamic finders.
Groovy, very similar to Java. Metaprogramming, easier JDBC, execellent, makes it very easy to work with JSON. Groovy also has performance close to Java. Java and Groovy can access each other as they have similar byte code.
Very few breaking changes from release 0.4 to 3.3. You can expect that most of your code will work without needing a rewrite the next 10-20 years.
Superb HTTP parameter binding and validation with Command Objects.
It is a very mature framework and you have full access to Spring Framework.
Great community!