From user standpoint?
Significant.
No any "modern" approach, let it be - reddit, twitter, hackernews, fb groups, discord, mailing lists maybe? gets even close when it comes to quality/merit of discussions that forum can enable
I agree that from a user perspective it can create very different "illusions" and (over the long run) even modify people's behavior - which is scary if you think about it: simple choices at the technical level can change society
added notifications on reply
removed three-like structure
and allowed threads to be bumped
then you'd have forum which would mean that you'd have longer and deeper discussions