First, the lack of a real threaded comment system. When replies below the first level all appear at the same depth, it becomes extremely difficult to have a real back-and-forth debate and nigh impossible to read/participate in someone else's discussion. It is not conducive to substantive dialogue.
Second, the lack of a dislike/downvote, particularly an anonymous one. When one shares something they agree with and only see that some (if any) of their friends like it, they might figure that the rest just didn't see it. Very rarely will someone register their disagreement with a comment because, even when you're receptive to discussion, a real discussion cannot be had because of the aforementioned shitty comment system. Worst of all, when you disagree with most of what a friend posts (and aren't the type to carry on pointless arguments all the time, or don't feel comfortable sharing your disagreement), having no way to easily register your disapproval, you quickly tire of seeing their stupid posts in your feed and the temptation is to unfollow them, further insulating them from dissent.
If people who post crap saw that many of their friends gave it a thumbs down, they might not think themselves right all the time. If you could hold a real back-and-forth discussion with someone and have it be readable by others, and see everyone's approval/disapproval of those comments, you might feel more inclined to participate. The conversations would better reflect the opinions of one's social group. There would still be some bubble effect, but it would be greatly moderated because hardly anyone has only friends who agree with them about everything and their friends who disagree would have a way to make their disagreement known by engaging in a substantive conversation or at the very least in the form of adding a dislike to their post.