How about easy integration into existing enterprise Java applications? Java and JEE (especially with Spring/Hibernate) are used all over the place, so Grails/Groovy offer an easy way to extend these applications using new tools, without losing all the benefits of the existing investment.
Familiar syntax and libraries in itself should not be considered as a small advantage. It is a HUGE advantage. Today one of the biggest challenge we web developers face is every now and then evolution of new frameworks which impose new syntaxes. This is not the case with Grails. You can be productive from day one. You can have part java /part groovy in your project initially and then convert to full groovy app, only and if only you want, at a later date.
If you worked with Hibernate and Spring MVC then the concepts are almost same under the hoods. Grails will just make your life easier by taking care of everything and leaving only utmost import thing - "your business logic" to you.