It's definitely part of the secret sauce, but I wouldn't call the moderation aggressive - it'd call it targeted to preserve culture. If MetaFilter has a first defence then it is the 5$ account wall, the second is the (almost legendary) snark, the third is the moderation. Moderation is almost always employed as a last resort, and it's rationale is normally documented.
The "secret sauce" of MetaFilter is really the community and everything else works to preserve that. The moderators of MetaFilter are the last resort in that process, but their are neither "aggressive" nor numerous, they are just very well chosen - not by algorithms, but by other existing moderators who actually understand the organisation/community they work in.
(Out of curiosity, do you know if there's any way to see how many threads are closed by moderators? My recollection is that the rationale for closing a thread is included in its comments, but the thread disappears of the frontpage once its been axed so it's hard to know how common that it.)
Either way, I didn't mean it as a bad thing. Personally, I want a moderated experience... so long as I trust the moderator.
I'm pretty sure it's been discussed in MetaTalk too, so some creative searching of that sub-site might turn up the number.
I really, really like that idea. Not just EFF, because that would tend to create an echo chamber. Instead, you should be presented with a dropdown list of various unrelated but preapproved nonprofits. A $5 donation to any of them would earn you commenting privileges for a limited time.
Want to comment as an AC? Donate $5.00 for a one-time comment without logging in. Want to comment as a logged-in user? Donate $5.00 for a month.
Somebody should totally do something like this.
Anyway, I think aggressive moderation is a big part of Metafilter's secret sauce.