For me it was the politics as well. And not even the amount of it, but rather the toxicity of discussions. Not that things were polite in elder days... but they were not so actively hateful.
I still wonder sometimes if it's a shift in the Slashdot audience specifically - seeing how movements like neo-reaction have been springing up in the tech community in the past few years - or it just mirrors the increased polarization in American society (I know Slashdot has international audience - heck, I was a part of it for a long time - but it's definitely US-centric).
Sadly nothing really recaptured the spirit of the first few years, and it can't. The internet isn't the Wild West any more, you're not using it over dial up from a box room staring down a CRT, times change, and you can never go home.