Anyone who's smart should be able to contribute w/o passing some sort of exclusionary bar.
This seems to have worked well for stack overflow. Just spitballing here pg, but what if you extended this to commenting as well? HN could be open to browsing, but to comment, you needed to be invited by another member with x karma? You could quantify this by requiring 10 karma to comment and 100 to post, and allowing a user to gain x karma by getting an 'invite' from an existing user with 100 karma.
Of course, without data, it's hard to draw conclusions about the fundamental issues.
If voting on a comment gives karma to the commentator and it takes karma to vote, then people would need to comment to vote. I think this type of zero sum game would increase the number of poor comments from people who want to vote and an increase in poor comments is what we are trying to prevent.
The difference is that to get even one or two stories a week on the front page of Digg, you basically had to devote your entire life to the site. Whereas getting 100 karma doesn't take more than a couple weeks of casual use.
Some of these people just create an account to make a few contributions on specific threads and then they're gone. Having an invite system would increase the friction in being able to do this.
I think that an initial 100 karma level for submissions will make the site more difficult to use for the not native English speakers.