You run the risk of alienating new members or them not seeing the whole of a thread and them leaving rather than getting involved.
The only way you I can see of implementing something like this is to have a donators/subscription area of the forum that is hidden from the public forum unless but you get access to it by donating and subscribing. It was done on another forum I was on years ago and the perks to subscribing was deals with ferry's, AA, etc, a board to talk away from others and an 18+ section for jokes and tasteful pictures that show more than our current women with VW's thread. Upside is the forum gets funds and donators/subscribers get deals, downside people might talk more in there than the public forum or donators/subscribers feel they are above "normal" members.