Plazas and warrens both have their unique set of tradeoffs. Warrens are notoriously difficult to get started. New users, stuck in empty warrens often don’t know how to connect to hubs of activity. The onboarding process is crucial and still not well understood (Friendfeed found that people needed to add at least 5 friends to have a reasonable chance of sticking with the service). On the other hand, plazas only need to be started once and then they remain a hive of activity for new users to participate in from the first day.
An interesting article on social network topographic models.