EXT-GWT Question:

Dynamic and reusable UI components: programmers can use pre-designed classes to implement otherwise time-consuming dynamic behaviors, such as drag-and-drop or sophisticated visual tree structures.
Simple RPC mechanism
Browser history management
Support for full-featured Java debugging
GWT handles some cross-browser issues for the developer.
Unit testing integration
Support for Internationalization and localization
HTML Canvas support (subject to API changes)
The developers can mix handwritten JavaScript in the Java source code using the JavaScript Native Interface (JSNI).
Support for using Google APIs in GWT applications (initially, support for Google Gears)
Open-source
The developers can design and develop their application in a pure object-oriented fashion, since they're using Java (instead of JavaScript). Common JavaScript errors, such as typos and type mismatches, are caught at compile time.
The JavaScript that the GWT compiler generates can be tailored to be either unobfuscated and easier to understand or obfuscated and smaller to download.
A number of libraries are available for GWT, by Google and third parties. These extend GWT's features.