Groups that work need:
Dedicated leadership.
A goal.
Watchers (potential members).
Members - people who agree with the goal and will be counted as such.

How these pieces interact determine the longevity of the group. The goal must be a statement of general direction (e.g. improving the Perl language and expanding it's usage). Leaders must be empowered to rebuke those in the group who incite rebelion to the goal(downvoting, remove a node), and to encourage those who work toward the goal(upvote and frontpaging a node). Members must be dedicated to the cause as well as evangelizing for the group. Watchers are those who have been evangelized, or happen upon the group by chance.

There must be enough freedom to discuss radical new ideas, on topic ideas, as well as some off topic thoughts. There must be a way to get feedback from those who count (leaders, members and watchers each weighted by proven responsibility to the group goal).

A method of allowing a person to move between the striations in the group. (watcher, member, leader - see levels within the Perlomonks). Rites of passage must be in place to let each member know their standing in the group as well as the standing of other members. These rites are important as the personal cost of each rite validates the members dedication to the group goal.

Once formed, a surviving group will be dynamic. Membership will change over time, with new members exceeding attrition. Leadership will change slowly as new people are added to the ranks of leadership roles, upper level leaders will share the position, take lesser leadership positions or step down from leadership positions as they see fit (dedication to the group goal). Changes to the core methodology of the group will also need to occur, but should only happen as they are proven to be beneficial to the goal of the group.

The goal may change, but then the basis for the group has changed and the original group will have to be considered dead (even if kept alive in name).

Thought was provoked, and shared. (a sense of humor helps most groups as well)

-John