- Better 3D rendering performance. I've heard Java OpenGL bindings are limited.
- Better community. The Java game dev community is pretty quiet. I'd like to see more activity.
3D rendering in Java is as fast as it can ever get.
As for community... this is it. It is what we make of it!
I actually have zero experience with Java 3D work. I really should have qualified that. There are Java 3D engines (jME, etc) but why don't we see anything that really competes with Unreal or Unity?
The community is almost non-existent weak. Cas, you are one of very few Java game devs. Even successful devs like Notch have jumped ship to normal C++ tools. This is a relatively fringe and disorganized group. Java games would need some larger sponsors to take off.
Cas, you should know: what else is limiting the success of JVM tools in game development?
I've worked with both Unity and jME. As for power Unity in many cases has jME beat, but not by much. I'm quite impressed by how well jME preforms compared to c++ based engines. As for making a game with the engine, I prefer jME every time. With jME you have much more controlled over the internals of the engine. With Unity, the developers seemed to do is choose the generally best algorithm, and lose any versatility for finding the specifically best algorithm.
This community is no where near quiet. It's not a five posts per second community, but I like how much this community contributes. I don't think sponsors are the solution. Look at mine craft, 0 sponsors and it took off like any AAA game while it was still in beta.
As for Notch's tools, I think they are all in Java. He just converts his launchers into os specific executables so more people understand how to launch his game.