log4j.appender.stdout.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n
The exact problem was that despite having that configuration, I actually didn't see any debug printouts emitted by one of the libraries I was using, just the info ones. The reason was that library used SLF4J and was bound to the SLF4J-Simple logger.
Since I had no time to go ahead and read through the whole SLF4J manual, what I did was removing slf4j-simple.jar and including slf4j2log4j.jar instead.
And all the debug messages were there.