Another criterion for FPed nodes is that they should already have a certain number of offspring nodes
A counter-example from earlier today: vroom posts to announce a new feature. Such posts are unlikely to garner many offspring nodes, other than the occassional well-deserved "you rock!" Should these be held off of the front page?
This is but one counter-example of many. Short lead-time announcement of things of interest (talks, PM meetings) are another class that often deserved to be front-paged immediately.
By the time you sort through the exceptions-to-the-rule, having a rule embedded in a mechanism (as opposed to in the culture) is very difficult.