The 3rd issue I find relevant is the amount of code had to be produced for the layout and interactive behavior of the application. The Swing-style coding is a real blocker. IMHO, GWT probably make heavier use of annotations, perhaps even introduce some custom annotations for simplifying the implementation of user actions (i.e. listeners as inner classes).
I found some comparisons of GWT, Flex and Echo2 on the web: one and two
And here's the quote I was waiting for:
I think this describes our case the most - a targeted audience.