It now looks like the fediverse is well on the way to becoming something mainstream-ish. I expect that over the next year the degree to which it succeeds will be the extent to which my timeline is able to resist turing into something resembling Twitter.
So far it's mostly been a success story. The last Twitter exodus in August did bring with it new challenges though. There was some kind of dogpiling scandal involving a minor celebrity and the lesson from that is that celebrities can bring with them a lot of toxic baggage from the Twitterverse which isn't necessarily trivial to mitigate against. Even on Mastodon it appears that people are easily enthralled by celebrity, and willing to give them far more benefit of the doubt than J Random Mastodon newbie would get.
Maybe other celebrities will be deterred by that debacle, but with the growing network effect we're likely to see a similar story begin again soon enough. It does raise questions about the extent to which the fediverse can avoid classic Twitterisms, like dogpiling of targeted users.
There was also an anomalously high level of squabbling between instance admins. Fortunately that only lasted for a few days and then things returned more or less to their usual condition. As I've mentioned before, the configuration of instance blocks kind of works itself out over time and settles into a new pattern.
One thing which concerns me is that the larger instances are continuing to grow without any limit on number of users, and that as they do so undesirable Twitter-like social dynamics begin to play out. I think what's needed is renewed emphasis on running your own instance and keeping the size of instances small. When instances get very large they become impossible to moderate even with a dedicated team, whereas with small instances peer pressure alone is usually enough to avoid bad behavior going unchecked. Small instances are a key advantage, since they're something which Twitter and Facebook can't do.