The existence of an index in which j is #4 behind tcl and racket should not lead one to think 'wow, j is really catching on' but rather 'wow, that's an unusual index.'
I think looking at Rosetta Code you shouldn't make conclusions about selling points :) . Rather you can use RC as a repository of J programs, which happen to have many examples - comparing to repositories for other languages on the same RC.
If you're familiar with RC, you know that it encouraged participation with unusual languages, as it's generally hard to find lots of examples of code for them. RC is a code chrestomathy site, but for Java and likes it's easy to find many good examples elsewhere. So rare languages comparatively shine on RC.
