Karma is probably a good metric. Getting to, say, 50 is pretty easy if you make a few good contributions to discussions over a few weeks (or a couple of really popular ones over a couple of days), and yet that's still much more effort than a spammer is likely to expend.
I just checked the users of the posts on the first page. All users that have (almost) no karma have posted junk.
Not all posts by high-karma users are good, but a base level of karma required for posting can't hurt. If somebody does't spend time on the site commenting to archive e.g. 25-100 karma, why should we expect that user to provide a lot of valuable links within the sites focus?
How often do you watch it? It moves faster during the day. There are times where there is more spam than not. And if you feature it on the home page, the amount will only go up.
Reddit did something like this before the top entry became a sponsored listing. Instead, the very top entry was in a blue box and was randomly selected from the most recent posts. Certainly got my attention and I voted a few up.
I'd prefer to see new, karma/age verified items in a second column on the right of the main page, rather than at the top of the highest rated items. This could be an effective way to use the empty space on the right.
Make the first 5 listings on the HN frontpage be the 'newest' entries, followed by the top rated entries.
This would force people to scan past new items and give them their "30-seconds" of fame. Plus it would keep the front page very fresh feeling.
It's like singing in front of Simon Cowell for American Idol tryouts.