If the motivation for this is content-related, why not something like "ignore users with less than X posts" instead? Some forums already employ filters that prevent you from viewing/posting certain things until your post count reaches some number. It seems to help with spam, as well as maintaining content quality. Ignoring works on the receiver's end (opposite to that) but it accomplishes roughly the same thing without annoyance, so I'm all for it.
1) It disincentivizes lurking. If the premise is "don't let me trip over you until you've learned the ropes" then simply observing for a certain amount of time will satisfy that criterion. On the other hand, fifteen Redditors descending and saying "DAE REDDIT!??!!???111" results in fifteen users with fourteen shared posts each. 2) The contribution of someone who sees much but says little will almost always outweigh the contribution of someone who says a lot. Your proposal punishes these individuals. 3) In a subreddit-like system, your proposal is indeed one way we control for spam. However, that's a moderation-based system whereas Hubski functions on a user-based system. I, personally, as the current moderator of one and past moderator of five default subreddits, hold much more faith in the healing power of time than I do in the multiplicative power of votes in conforming a user to the prevalent culture of a community.why not something like "ignore users with less than X posts" instead?
Your second point makes sense, but unless I'm misunderstanding, wouldn't a time-based ignore also do the exact same, though less strictly? Considering that the lack of moderated 'categories' already discourages much of the current issues on reddit, I doubt an ignore feature would do much good here; I merely mentioned it as an alternative solution, if one is ever needed.