You need to also change the default logging level of the appropriate handler (the defaults are "INFO"). So, for example, if you wanted to log to console you'd change the following:
If you're using 4.0.0.Beta2 with DEBUG logging enabled and using binary values, you'll also probably run into: [MODE-2302] The FileSystemBinaryStore incorrectly handles the binary unused / used cycle - JBoss Issue Tracker
This is something that will be fixed in the next release.
Strangely enough, I had set this CONSOLE parameter to DEBUG.
I restarted Wildfly this morning without having touched the configuration file... and I got DEBUG messages indeed.
And of course the original issue I had didn't materialize anymore.
Was it some kind of LevelDB-related "stray process"?
Anyway, I'll see if that pops up again.