Comment by InclinedPlane

I pointed out something similar in another comment but it's worth focusing on. One of the biggest problems of both reddit and slashdot is that they embrace humor at their core. This may not seem so bad but it's poison for mature conversation. Humor is cheap. And it doesn't require engagement.

What you see on both reddit and slashdot is an increasing tendency toward irony as the basic approach to any subject (4chan is the same way too). This overwhelms and actively drives away discussions that are serious and not ironic. This is why the highest quality subreddits (like askscience and askhistorians) enforce very strict rules and actively discourage humor for humor's sake. This is a big reason why stackexchange is so successful as well. And it doesn't mean humor is unwelcome at all these places, just that it needs to be part of something productive or humnorous enough to overcome the strong bias against it.