It's not necessarily slashdot's fault the, for lack of a better word, "quality" of the commentariat has diminished. Basically, /. in the late 90s/early 2000s occupied a unique place in time that won't really happen again. Today, the experts have naturally dispersed and the noobs have naturally shuffled in. I don't think there's any place really "better" because the world is flat. Some blame Dice, but it's really just the fact that the landscape of the internet has changed. Knowledgeable people who wanted to discuss tech, free software, science, etc have many more options for places to call "home" these days. It's the same as the Eternal September on Usenet, except that change was dramatic, but this has been slow.
I started reading /. in I think 1999 or so but didn't make this account for quite awhile. Back then...discussion websites didn't really exist much. The concept of an open "community" was just not very common. Yes, there were forums and things. But not content aggregators with lively discussion in comments to articles. And what was awesome about /. was you would be reading comments about an article about particle physics, and just organically one of the people in the discussion was a particle physicist. It wasn't like they "brought one in to answer our questions" or something. He was just...a regular /. reader.
And the self-selected population would naturally have a more than passing familiarity with physics to make the discussions worthwhile and to make the questions they asked the particle physicist meaningful and worth his time to answer. These people were on /. because there were not many other places to go. Back then not everybody was on a computer all the time. If you were on a computer, on the internet, during the day, often enough to regularly read and post on /., you are likely to be some kind of a technical person, probably with a technical job.
So since everybody likely had a technical background, people who asked stupid questions or didn't understand what was under discussion and posted stupid shit like "LOL I don't know what any of this science stuff means!" didn't get their hands held. They got told to STFU, RTFM, or GTFO. Is that appropriate everywhere? Of course not. But it's nice to have SOMEPLACE like that.
Today you can go to reddit and discuss physics on /r/physics. Or Linux on any of the myriad other Linux discussion websites. Today, everybody's on the internet all the time, so it's noobs, far as the eye can see. There will likely never be another year 2000 /., just like there will never be another 1992 Usenet.
That was a rambling bunch of nonsense, but there you go.