Slashdot - aggregating opinions from "experts". allowed comments. Lacked accountability so then came...
Digg - allowed crowd source voting. active interaction. complicated and over-engineered upvote algorithm lead to...
Reddit - community inclusion. more open voting (and down voting). subgroups.
So that's kind of where I see it. What gets improved on? Is there technology that can be improved? Is there a missing idea or improvement? Is there a next generation that could improve status quo?