Slashdot had a strong community and good stewardship when it started. Eventually it got bought (and I think changed hands many times) and became a thin commercial shell of what it once was. In the mid 2000s when I used to read it, there were some (many) good technical discussions that would be similar to what you would see here. The two sites are just different places on their arcs. Check back on HN in 20 years.
To add in response to the post, there is clearly a sweet spot of how many users a site has. At the beginning, fewer genuinely interested people can work, and it can get better as the number grows and more people contribute. At some point, you have too many people, and the combination of eternal September effect plus not being able to please everyone plus trolls and people looking to get upset about something means the quality goes down (that's where a lot of reddit is I guess). Then when numbers go down again, a lot of the quality users leave and you're left with the trolls etc. so that even if active users is higher than at the beginning, the quality is gone. That's where slashdot is.
To add in response to the post, there is clearly a sweet spot of how many users a site has. At the beginning, fewer genuinely interested people can work, and it can get better as the number grows and more people contribute. At some point, you have too many people, and the combination of eternal September effect plus not being able to please everyone plus trolls and people looking to get upset about something means the quality goes down (that's where a lot of reddit is I guess). Then when numbers go down again, a lot of the quality users leave and you're left with the trolls etc. so that even if active users is higher than at the beginning, the quality is gone. That's where slashdot is.