that said I did once have a fiendish (and probably terrible) idea for a similar system where you could only post if you had a certain amount of karma, but also, where you could buy karma points outright.
Alternatively, there could be a time period before users can post comments or articles, though I don't really like having a time limit for posting comments, since that hinders insightful comments from throwaway accounts. It would also hurt users that only want to post because an article is extremely relevant to them and they know the subject well (or are involved in it). Really no winning scenario to this.
We could have some sort of user "turing test" to see if users are ready to post/submit on HN, but then one is left with answering "how does one know a user is ready and what are the qualifications?" On some of the Android forums where every user (hyperbole sorta) that roots their phone thinks they're a development expert, I always wanted to implement forcing users to look up various parts of the AOSP source (and/or answer questions about terminology) before they could comment in the development specific subforums. Though that is a bit more black and white than something similar applied to HN.