Code splitting to boost up performance and scale up your project is something I would love to work my ass off to learn how this code works. It plays a vital role on keeping my users out of boredom.
I am also looking forward to work often with draft compiling as I remember fixing minor tweaks consume a lot of my time.
These are just some of the features that I am very much interested but still the new features GWT 2.0 is very useful and I would be very excited to learn the new features and take advantage of it.
I personally recommend to move all your web application to version 2.0
For more information visit:
Google Web Toolkit
What's New in GWT 2.0?