What was the timeline again... Slashdot emerged in 1997, it developed voting on several different criteria, you could filter for "funny" or "insightful" posts, and it had a pretty sophisticated meta-moderation system.
Digg simplified that with up- and down-votes.
Reddit borrowed digg's model.
I'm not 100% sure what the innovation of reddit was at the time, but I feel like these three just had slightly different tones to their communities. Slashdot was the geek site, Digg had more jokes and politics, reddit was sort of... deliberative? The culture was a lot different back then, a bit more like this site.