the AOPA UK website doesn't explain the fee structure, but this was because it seemed to cause so much confusion when we did publish it and the page is cleaner without it.
I looked at the downloadable membership form which you pointed out and was frankly quite shocked at the disparity between joining this week (end December) and next (January).... membership through end March 2012 would cost £144 today but only £77 next week... not at all obvious from the membership webpage itself. I can see why it causes confusion and why the scheme so desperately needs updating. I'm sure this must deter prospective members from joining - it certainly has caused me to think twice.
I've done some more research and read various views on AOPA and its operation here, here and here.
It seems to me, as a complete outsider/novice that AOPA is worthwhile of support by almost every UK PPL, although less than 20% have signed up. The primary reason for joining is to support the lobbying work done by its representatives, but it has achieved some significant contributions to GA in the past (including development of the IMC syllabus and qualification in the first place). Mike Cross gave a good summary near the bottom of this forum thread
The replacement membership scheme was announced earlier this year in GA magazine, indicating that crew cards will be issued to all members and that annual membership fees will be payable on the anniversary of joining. You have indicated above that this scheme won't come in until after the next annual membership renewal in April 2011, but I've not seen any firm date for its introduction.
In the spirit of providing some constructive feedback (I appreciate that a lot of AOPA work is achieved through voluntary contributions), I would encourage AOPA to expedite the new membership scheme as quickly as possible, publishing a target date for introduction. If this is within the next 6 months, consider adopting a transition arrangement whereby only 1 year membership is offered for £79 including crew card (valid through end March 2011). The 3 membership categories (pilot, instructor, associate) should be explained on the website with the strong emphasis on pilot category (who surely form 90%+ of members). The headline figure of the annual membership fee could then be shown clearly near the top of the webpage and the paragraph about crew cards would then be valid for everyone.