Profilers, reduce package complexity
The first item that always hinders my adoption of a new JDK is a stand alone, GUI driven memory & profiling tool. Yes netbeans has jfluid, I use vim / eclipse / notepad / intellij, don't hold that against me. Yes the open source community has things, but I can't trust a lot of them initially for commercial purposes. I know you have them sun, you wouldn't release a vm without those tools, so cough them up and help us adopt a new JDK.
Second, I agree with the general consensus to clean up the deprecated packages. While Java is a language, it's also written in code that should follow good design principles that Sun has preached (even if Joshua works for Google). "Item 12: Minimize the accessibility of classes and members" I'm not saying be radical, but I think going from 1.0 to 1.1 compatibility is not unreasonable. There are a few reasons I can see to support this ...
1. Microsoft is losing ground on the browser, and 18-24 months from now I expect it to be a dead heat. Where do you think people get their vm plugins now ? (sun)
2. I don't believe 1.0 compatibility is very important, but I could see 1.1 with IE compatibility. I can live with baby steps.
3. Make the language easier to use. That was the theme in JDK 5.0, extend it in JDK 6.0 by reducing the number of deprecated methods. I think ease of use should be a permanent theme in future releases, and reducing package complexity would be helpful. (although not as 'sexy')
I agree with many of the other suggtions in this forum as well, so I think Mustang could find itself not unlike tiger in scope. :) Good luck !