I think the point to return trust to Java is the most important. Currently, many users that read about the need to install Java go away and look for other games that doesn’t require them to install anything. I think that most of them are just afraid to get some weird thing on their system as they didn’t hear much of “Java”. If we help to make the name “Java” as popular as “Flash”, more people will dare to install JRE to play Java games. And promoting a resource like JGT which specifically mentions “Java” and offers a great bunch of only Java games will help all of Java game developers - more people will come to JGT, most likely they will find a game they like there as there are many games and they will install Java. Later, when they come to your Web site, they will have Java already and will be able to try your game right away. I believe that the scenario I described is quite possible and it is worth to support JGT for the sake of the “Java” popularity.
Another idea is to create a site like Kongregate but entirely focused on Java. All the games would have to work as a Java Applet. The game site would provide a template, e.g. a GameApplet. The requirement would be something like Java 5+.
It would guarantee that all the games would work once they use the GameApplet. The site could provide a chat window, rating system etc.
Obviously the original developer will keep the full IP. The games on the site would all be free (probably demo/lite version). There would be a link to the game’s/developer’s full version.
Now, I am somewhat doubtful to get enough support. On the other hand, java4k.com is a great start. Limiting games to Applets & opening up the limitations to >4K would probably do the job?
how about a java application, that works just like “steam” but for java games.
wouldnt me suitable for webstart games, only downloadable ones.
games would also have to conform to the menu system.
Non free games should be purchasable with credit card through the app. So whoever develops the game manager would take a small percentage.
The only way it would take off, is if some games required the user to install it. Just like HalfLife2 required steam.
I think this would work, the only question why would it need GameApplet? I think that the Java 5+ requirement is quite enough and it should be the only one. And a site like Kongregate with the properties you described seems to me a good option to promote Java.
Yeah, limiting games to Applets but letting them to be of any size will open the doors to many games implemented as Java Applets. And if there is a rating system that allows the user to only see games of a specific category, e.g. “recently added”, “good rated”, “all”, - that would make it easy to use and attractive. In my opinion, any Web resource that contains so many games that even any visitor can find the one he/she likes will become popular. And if this resource forces the users to install Java, this will make Java itself popular and more spread.
Java Game Tome is a resource of this kind. Of course, another one would make it even better but it doesn’t seem that it can appear soon as there is not much optimism regarding this idea 
Meanwhile, I’ve added a back-link to JGT on my Web site
Java4K.com is dedicated for the 4K games contest, not going to change.
There are all sorts of problem associated with creating a full fledge kongregate-like game portal. Why Kongregate.com has full-time staff working on the site is self-explanatory.