Two are quite up there, Friendica and Mbin’s hybrid philosophy, and if an instance goes “poof”, posts and instances already federated keep floating around. Also adjacent to both, boosts and following users, which both engines have even for threads and that better allow propagation.

Also a grievance I have with Twitter but that I like competition still uses: tags
Much better than the current copypasta meta, or whatever is the current one for making things relevant.
Also helps finding spammers more easily for blocking them. :v