What is the Log level that should be set to see user-generated debug statements?
You can see user debug statements with log level "Debug" or higher:
Debug, Fine, Finer, Finest, Internal.
You don't see user debug statements in levels "Info" or lower:
Info, Warn, Error
The minimum value to see any debug logs is a System level of ERROR, but most people only use the default logging level, so the usual recommendation is DEBUG. To debug at a lower level than debug, use the two parameter version of System.debug:
Also note, logs over 2mb may be truncated, including your debug messages.