Maybe a counter at the top of the page (karma, counter) to show how much time someone has spent on HN in the past (day, week, #{interval}) guilt-trip (some) people into spending less time on the site? Or with the number of times they've opened up a page to HN?
// edit: This is a social/mental problem, not a technical one. I'd be surprised if a complicated change did anything to fix it. Noprocrast works because its simple. Changing algorithms around isn't as obvious, any behavior changes as a result would probably be much more subtle to observe.
The only time it's a productivity hit to me is when I'm procrastinating anyway. I wouldn't worry about that. Focus on making the conversation as interesting as you can.
As a few people have already pointed out, a lot of our time is spent consuming the content and not necessarily creating it. Reading the comments is probably the biggest productivity hit for me. The reason for that is that you can quickly get stuck reading long threads of discussions - especially on the more popular articles. While this is great, maybe try making only the top 5 or 10 comments visible by default, and then giving the user a 'See More' button that would show the rest of the comments. You could also only show the first and second level comments in the default mode, which would add to the simplicity and make it less of a productivity hit. It would be interesting to see people's reaction to this feature if you ever decided to implement it.
Personally, I dislike the "more" way of viewing comments. I like to scroll with the arrow keys or page up/down (well, fn + arrow). If I have to move the mouse over to "show more," I'm just as likely to hit cmd + w. Especially if its "more" for different levels of comments and to get more comments. The button won't even be in the same place!
Make it so that comments don't need to be made right away (to get upvote exposure) to be in the top 10 or 20. Maybe you could normalize each upvote by the number of views per unit time (view rate). You could measure success by trying to increase the average age of the top 10 comments (age = time since article was posted).
What if you emailed a daily digest of the top stories? I visit hn less often when I get the content elsewhere. I found the twitter bots that post links help with this. But twitter is itself a time wasted. Email is easier to control.
Or, what if you reported on the time spent on the site in a give day. Make the font size num_hours*4 em. You make why you measure, right? Show a graph of waste and people might try to make it go down.
Have a simple graph on the profile page showing how many minutes the user spent on HN each day in the past week and/or a number on the top panel showing minutes spent on HN over last 24 hours.
I find that just measuring something is a far more powerful way to improve my habits. I am far more likely to set noprocrast=1 if I realize I spent 3 hours on HN yesterday.
Edit: Just realized Zev had posted the same idea a while back.
Have a widget on the homepage that shows the top 10 links of the day so that we don't have to read and sift through every single submission. Just reading only 10 links per day would be less of a time sucker.
I don't read it that much at work, but do more so when I am working at home, and sometimes on the train. It is my go-to place to find out what is going on.
