I'm sure you could calculate rankings based on post length and other such things; the only limiting factor would be someone actually writing the code to do that
Don't know whether Qhimm or SaiNt would have the time to do it.
Ranking based on posts usefulness is fairly cool, but the only way to do it is a self-rating system; the boards members each rate other peoples posts. Usually it'd be combined with a scaling system, so the higher rank you are, the more your opinion counts for than other people's (though not enough that it makes theirs useless, just a slight weighting difference).
If you're going with automatic ranking, the sensible thing to do would be:
-Ranking based on length, penalties for one or two word posts, bonuses for long ones (unless it's a *useless* long post of nothingness, in which case the mod steps in). Also not counting quoted text.
-Penalties for posting in a topic when the last person who posted was you. Chains of 8 messages all from the same person aren't very nice (the Edit button is good, people!)
-Still tracking raw post counts for the boards stats, and maybe users' profiles, just not calculating ranks directly from them.
