Perlmonks certainly works. However I don't think that the level system is the reason. My opinion is that the real reason is that a good community was established early, and then a focused remit maintained it.

Ironically that is one of the reasons why I lost interest. My interests moved on. A small fraction of the conversations taught me anything interesting. And so I drifted away. However the focused remit is essential for maintaining that community as it is, even if I am no longer that big a part of it.

Incidentally I am http://www.perlmonks.org/?node=tilly there, and http://www.perlmonks.org/?node_id=754085 describes some of the early history of how it came to be as it is.