Separate activity points and spendable points.
I want to encourage activity on my site and spending of points. I don't want people to get discouraged from spending because they'll lose ranks and privileges by using their points. Instead, there should be a total points function that counts for ranking and prestige, and a spendable points count that shows how many points the user can spend.
Also, it would be cool if ranks were only activity based and the user couldn't buy ranks. They'd buy spendable points that they can use on all the spendable features, but this wouldn't count for their activity points. It discourages people from commenting if they know someone else could come in and buy their way to the highest rank.