[quote=@BBeast]
A limitation of "x posts to graduate" is that someone could spam out posts and graduate, in which case they'll have privileges until they are caught out, at which point it may be too late. Of course, I'm sure you have already considered that.

How about a minimum time AND post requirement (not just OR)? This means participating members of the community who have been around some time would be promoted. Of course, you'd include your more sophisticated factors too.
[/quote]

Or only allow 'New Members' to post once per x minutes, and if they keep posting exactly x minutes after the last it's very likely they are a spammer trying to game the system, and it can be detected and handled appropriately.

EDIT: Or maybe require that everyone does a captcha before being graduated into the next tier, or at least for the first graduation. That would filter out all the bots, at least.