Groovy/Grails works well with GWT. Idiomatic Groovy looks more like Python than Java. Another possibility is Vaadin, which is built on GWT.
Also i have found with vaadin the client side rendering times can be a problem. ( though my whole team is new with vaadin so there's a good chance that that is our fault)
I definitely see the appeal of vaadin, but i think that ultimately its not the paradigm you want for web application development. I think that for many cases you really need to know exactly what will run on the client side and what will run on the server.
Back when i was doing lots of silverlight development there was a very clear division between the client side and server side code. However, because they were both implemented in the same language i was able to factor out shared components (models mostly) into seperate projects that i could individually compile for both sides (for smaller chunks of functionality such as some socket protocol validtion code i just linked the files into multiple projects). This allowed for good reuse as well as good seperation. I think that this is ultimately what you want: the ability to share some code between both sides not to blur the line completely.