I really don't think it has as much to do with content as people are expressing.
Content, as far as I'm aware, was only a recent addition to the community supporter account benefits.
Although I think offering a $3 a month option was a solid step-forward, removing the other payment options was a major, major step back. As has been mentioned, $3 a month causes many people to constantly 'review' their decision to subscribe. And as has been mentioned, many people would take up the option to simply subscribe for a longer period as it's more convenient and cheaper (if you have the upfront cash to do it in the first place).
The very first thing I would do, before anything else, before reviewing content, or offering more options to pay other than PayPal, or even before investigating any further, would be to reinstate longer subscription options. That alone, I believe, will help things improve fairly quickly.
I'd recommend something on the order of $3 a month, $16 for six months, $30 for 1 year. Don't look at it as "1 year is $6 less than I'd get if they were $3 a month", look at it as, "That's $27 more than I might get if they were reviewing their decision after a month." Do the same for the gold, silver and whatever levels.
On a personal opinion level, I'd also scrap the entire notion of subscriber only content and board features. It's no doubt a headache, not to mention time-consuming, to manage all the different levels of access. Either do content or not. So far I'm getting the gist that doing content is not profitable, so personally I'd scrap it.
Make the boards searchable with full features for everyone, whether they're a subscriber or not. Keep the "gold, silver, copper" (or whatever levels it is now) but give them non-essential perks, like a gold, silver or copper icon next to their nick or something. Whatever it is, it shouldn't be on the level of getting something for your money. You shouldn't have to pay for things on a forum.
This, I believe, will help engender and build the community as a community, rather than as, "what do I get for my money?" People were donating to the site before it ever required donations for perks, and people have donated to the site many times since for no other reason than to help keep it alive. People will only keep being generous as long as their IS a community to foster. People's generosity doesn't hinge on the ability to search or download an adventure module.
By not tying subscriptions to content, it eases your burden by a big amount, and ends the whole notion of supply and demand. People will continue to support a community they love, the problem here is that for many people, it's becoming a community they no longer want to support.