Personally, despite the frequent requests to reduce the donation amount required for a rank I've always respected the fact that it hasn't happened, and the stance has been relatively firm.
*~$100 (BTC didn't got below $2 since theymos started accepting donations in return for Donator and VIP titles).
Sometime i feel that forum is missing a lot of funds by not using google ads. With the sort of traffic, this forum generates, google ads could generate a lot of revenue for the forum, but then it will not remain as clean (ads free) as it is now.
Bitcointalk.org is primarily a discussion forum, not a business ran to maximize income. There are many reasons against having Google ads, such as: privacy, preferring to have more control over which ads are shown on the page, and not wanting to censor more content than necessary just to please Google (to be compliant with their gambling policies, among others).
I'm one of these people who have started to boycott some big companies including Google as I find the way they handle things, and specifically the privacy concerns to be too much to justify using their probably superior services to their competitors.