I don't think there is anything wrong with awarding points a la frequent buyer program (i.e. purchases, subscriptions, even number of reviews posted, etc.). I think you should spell out the rules clearly so people can make informed decisions (MontBell is a good example), and minimize any feelings of being played around on our part.
I also think awarding points based on "forum activity" or something similarly nebulous (to the extent it remains undefined) is counterproductive and may even be a "turn off" for some.
Finally, I am sure you already have means of tracking "premium subscribers" without needing to brand them next to our "handles" each and every time that we post. I would rather not have this lavish display, for the same reason that I don't wear THE NORTH FACE apparel.
In lieu of this public display, I would much prefer a way where we can individually query the number of points we have accumulated (i.e. individual accounts).
Thanks for reading.