Seems like an unwelcoming system for new users when (I assume) what you want to tackle is the very small number of spam posts. A bad side effect could be that

Seems like an unwelcoming system for new users when (I assume) what you want to tackle is the very small number of spam posts. A bad side effect could be that HN users might be reluctant to post interesting items that were only likely to appeal to a minority of their fellows. Technical items on web development rarely get more than three or four up-votes and yet many are likely to be of interest to web based start-ups - part of the core HN target group. Does not feel like a net win to me.