Muhaha (looking at the results so far). It's way too late. I pretty much got on board the moment it started to go downhill - same time the reddit crown (me included) came here.
Communities like this have two big problems to solve when they grow up:
1. "The crowd" comes in, and dwarfes by sheer numbers the original community, pretty much diluting the original values to a fraction.
2. Same community, same members, same ideas - after a while you get stuck in a loop, and it's hard to bring truly original content.
Unfortunately these two problems don't solve each other, but rather make things worse. The crowd will assimilate only the most superficial and more repetitive values, while the true target for original content is lost pretty fast.
What I'm saying is things are pretty much doomed, and I'm not talking only about HN. Believe it or not, there was a time when digg was better then HN is now.
As for solutions, I have none. But do like the idea of invitation only... maybe with a twist. You still need to be able to get new members directly, not through aquintances. So how about having "full members" and "regular members". Regulars can't post stories, and their comments are somewhat at a disadvantage (graphically or by initial points. I's suggest both). Their comments have "invite" buttons visible to the full members, possibly with a window showing comment history in between. Full members have a number of invites logarithmically proportional to their karma.
To the HN today it's both too late and impractical to apply this, but a clone could be worth it. Maybe starting with the users with the most karma as of one year ago.