• Timing/luck
• The people
• The XP/level system

PerlMonks was right timing-wise to position itself as the forum that isn't Usenet or mailing lists.

It had cred from the start in the form of well known Perl people. It also attracted enough not-so-well-known but still expert Perl people to write high quality posts, both on the topic of Perl and on software development in general.

The XP and level system is what provides stickiness. I think that especially the fact that you get XP for voting (and that you get new votes every day) makes new people establish a pattern of returning.

For existing users, it's easy to keep up with what's new using e.g. Recently Active Threads (which unfortunately isn't obvious enough for new users).

/J