I honestly don't think this will diminish user adoption, or activity. But there is only one way to know for sure.
Change for change's sake...? Rate of new users joining seems fine to me -- why mess with it? But you don't have to justify this to me; you've spent a lot more time thinking about the pros and cons than I have. I was just registering my opinion in case this was a straw poll.
Not for the sake of change. I want to enhance the expectation that goes beyond: "Sign up and drop a comment." As an example, the Reddit influxes always result in a disruptive education period, and the burden falls upon the extant users. cgod often ducks our for several days, and kleinbl00 has begged to have a newbie filter. IMO a reasonable barrier would require some reflection, and beg a more thoughtful entry. Also, it will combat spam.
Heavy-handed way to deal with spam. The reddit influxes always work out pretty well. I hate them while they're happening, and I can't remember if I'm following the nearly-pointless #askhubski right now or not, but the aftermath is an improvement in the hubski brain trust. It's possible that, based on stuff I've said in the past, you expected me to be pro-invite only. I'm a member of a lot of great invite-only websites, and I'm an outspoken proponent of this axiom. And indeed somewhere down the line I think this policy is not only positive but necessary. But for now, we don't have a growth problem. We don't really have a content problem. (Or the one we have isn't going to be fixed by an invite system.) We have muting etc, which I would love for you to make even stronger. EDIT: if you must, maybe a new-user filter is a good idea. 24 hours, say. It's self-selecting; anyone patient enough to read-only for a day will probably contribute well afterward. Under your suggestion, how do good new users join the site? How does a random internet user who happens across hubski ever join? Bookmarking until the 1st of the month, then remembering to join? Won't happen. Getting invited? Not sure how that would work. I can invite people I know, not people I don't.