Lobste.rs definitely feels like less of an echo chamber: both Reddit and HN have a large "I don't agree with you, downvote" population - on HN this is officially endorsed as "correct" use too.
The biggest example of that IMO is people presenting opinions as fact rather than something to be explored. This can be subtle, and in conflict with how people debate or often discuss between two people, but given it's a public forum, a sprinkling of "I think" or "it seems to me" in in comments goes a long way towards not only not triggering some people into down voting, but also seems to promote more useful discussion around the topic as those or others seem more likely to voice their differences.