This is rough back of the envelope calculations, but we only have 62k paid users, only about 12k come back every day, so subscriptions would need something like 25%-50% of the daily userbase paying, not just 2%.
I just tried signing up for the site. My first hurdle was finding the sign up button. It's nearly impossible.
After finding the "New user" link and clicking on it, I'm presented with your community guidelines, which I understand make MeFi what it is. But, that page could still do with a sign up button, instead of the link buried at the bottom.
Anyway, I clicked on the "Go ahead and sign up for an account here" link, filled in my details and then I was presented with the "pay $5 to complete your signup" message. I didn't know it was going to cost me $5! I went back to the guidelines page, and I noticed you did mention the $5 there, but I didn't read it.
I expect this is the most common user experience of new visitors to your site interested in joining.
I don't know if improving these things will move the needle at all for you, but there seem to be a few simple things you could try to increase signups.
Lurking on Metafilter doesn't require an account; you need one only if you want to post or comment. The $5 threshold (and the wall of text "guidelines" page where it's mentioned a couple of times) serves as an incredible gatekeeper. That is probably the single most effective thing that is responsible for the high quality of Metafilter's posts and comments. The exceptionally well-done moderation is a very close second.
Most mefites actually did read the guidelines page, because by the time they've decided to become a member, they know that text on that page is probably important, and (much like the rest of Metafilter) is probably worth reading in full.
Because what usually happens is a user will read Metafilter for days or weeks, slowly realizing how special it is, and then finally hit a topic that they're passionate about -- the kind of thing where they just have to post, because they know they can contribute to the community... so they spend the $5 and sign up.
The catch is, they've been a member of the community for a bit already, albeit a mute one, and they've probably picked up on some of those guidelines already. That's the point. Optimizing the site so a first-time-visitor is more likely to become a (paid) user would inherently be deprioritizing community quality.
It's that community that makes MetaFilter what it is.
But just wanted to say that I love MeFi but the interface leaves a lot to be desired. Really :(