This was originally going to be a response to thenewgreen in that other thread about this, but the discussion has advanced beyond me, apparently. All of the following are my personal thoughts on this idea, and Hubski in general. I wrote this while I was very tired, so the flow of some of the ideas might not make sense. Don't kill me, please.
Hubski is a scary, difficult place for a new user. I was a lurker on Hubski for about a year, maybe before joining? And I wasn't getting an inviting image out of it. It was an interesting place to browse, but my biggest turn-off was the user-base. I felt then, as I feel now, that Hubski was the first old boys' club I'd ever seen that wasn't a country club for the rich. There is a pervasive feeling of general smugness and snootiness (Isn't that just the funniest looking word) among the people here.
At first, I didn't know why I felt that way. Everybody here was at first glance nice enough. The conversations were polite, ish. I kept reading and rereading comments and posts that I got that vibe off of. They had several things in common.
1: A veneer of kindness kept up until a disagreement.
2: A condescending and sarcastic tone.
3: A sense of superiority.
So many discussions are filtered by these problems that any large thread has multiple instances of them. It makes the conversation difficult to join, and uninviting because of it.
Next comes the hard part. The people here are, on the whole, not so bad. I've had arguments with practically everybody here and the majority of the time I was the source of any deucery. But there is no patience here for differing opinions. Lip service is indeed paid to the idea of tolerance, but any dissent is punished with condescending sarcasm. This becomes almost omnipresent when dealing with any speech deemed unfit by the "quality" standards of Hubski. These standards are enforced with rigor by people hiding snark under the guise of keeping to an arbitrary level of quality.
Hubski, as a whole, is one of the few places on the internet where it is possible and common to develop friendships with other users. This is made possible because of its size and popularity. Hubski has a small userbase coupled with a small amount of new users. This prevents "Eternal September" because new users can be ingrained with the site's social norms faster than other users can join. However, it also leads to viewing prospective users as inherently troublesome or useless, as time must be spent to rein them in.
So where the fuck am I going with this? Good question, strawman listener. An invite-only system only works when many, many people want to join a website without being familiar with its culture. This isn't happening. Not only is it not happening, the culture and nature of Hubski actively turns away new users from joining. The "secret club, us-against-the-world" mentality, funnily enough, makes Hubski a thoroughly difficult place to enter. The only time a flood of new users appear is when Reddit decides to blindly send a few hundred people all in one day. The keyword there was "blindly." An invite-only system would be appropriate if that was a constant situation, but each Reddit Rampage only lasts a couple weeks, with months between each one.
My point in all of this is that there isn't a point to controlling the number of new users. Hubski does a very good job of controlling growth all by itself. Sure, if the popular opinion is to do it, do it. But it will only hurt in the long run. When the issue is having too few people at the party, don't put a lock on the door.