GWT reduces complexity for this sort of thing imho - the complexity comes more from the way the client and server code are tied all together, the rather excessive abstractions and factory processes, and some legacy stuff.
GWT should be just a pretty simple client that crosscompiles nicely to browser specific, very optimised, js.
I have been wanting to do this for ages...but without a established client/sever protocol its a nightmare.
And GWT is fine, I love GWT, but the current codebase has the current client and sever tied together you cant work on one without the other!
They badly need to separate them out, establishing a protocol in the process.