Number of posts would tend to have the effect of increasing trash posts, people with the one liners or one worders just trying to up their count.
Membership length would have the same issue, someone who's been registered since the last forum rollover with six total posts over that time frame probably isn't intellectually invested in the site.
I decided a couple years ago not to give Rich any more money for Fauna, if someone really feels like digging, they can probably find a post or two explaining my reasons but in the end, they are just that MY reasons... so I'm S.O.L. on the new system (although I'm disinclined to use it anyway, same as the karma), but I don't think there's any other impartial way of determining which members are or are not invested enough to get ability. Length of membership doesn't work, there are long term non-posters and probably any number of multiple logins that have been long dormant. Total post count doesn't work well either because it would both encourage and reward hollow posts (with a flat number, a post that consists of "lol" would have the same net value as someone who took the time to write something meaningful on a subject). Only thing left is contributor levels.