Maybe the more you donate the more perks you get? Make a few tiers of donations, and just make it so that the higher the tier of donation you choose, the more perks you get.
As for what perks, I don't think board exclusivity is a very good idea. We're all friends here and I'm sure whatever is going to be discussed in the donator-exclusive boards will eventually be "leaked". =P And if it's something as important as new features, then I think it wouldn't be right to exclude other members from sharing their opinions on it.
Extra permissions sounds right, although I'm not exactly sure which permissions would be good as donation rewards.