The requirement for not only the standard captcha and confirmation email, but also for an administrator to manually check off on any new accounts, seems likely to cut down on active discussion in these forums. I know that when I started creating my account, I planned to comment in an ongoing discussion, but I don't even remember what that discussion was any longer, let alone what I wanted to say. In most forums, I can go from registering to posting in 5 minutes or less.
I've run forums, and mailservers, and mailing lists, so I'm aware of the huge problem spam presents. Having run them in the past, it seems unlikely that it will help much; the captcha and confirmation email are usually enough.
The goal of spam prevention measures is to stop spam while blocking as little ham - that is, legitimate messages - as possible. The requirement for manual approval seems like a step too far.