I'm not sure that an annual fee for all members is going to work. I'll bet forum use will drop significantly if you decide to impose a mandatory fee.
On the other hand, gixxer.com (motorcycle forum for those not in the know) accepts donations which grant access to special segments of the forum and gives a red R to those who donate (GSXR). I'm not sure what percentage of the gixxer.com population does pay for it.
Over the years I have been a member of a number of forums depending on my hobbies at the time (Acura forum, Infinity Ward gaming forum, GSXR motorcycle forum, HIDplant lighting forum, 4Runner forum, Mr Excel.com forum, etc). Aside from gixxer.com, I there hasn't been any other forum which requires a fee to be a member.
It's nice to quickly get an answer to questions about most anything from any/ all of the forums I've used and hear/ share stories both good and bad, but frankly, if I searched the internet for 5 minutes I'd be able to find pretty much anything.
All that said, I'd be willing to pay a fee is it was reasonable. Gixxer.com donations start at $5 I believe and are not capped. I'd imagine a lot of people will feel like the need to get something special in return for their money, not just access to a forum...