> Three, vice versa, Mastodon and its users rejecting the culture of non-Mastodon projects whenever it differs from Mastodon's. Many Mastodon users want everything that is "un-Mastodon-like" banned all across the whole Fediverse because it disturbs them, be it posts over 500 characters, be it quotes, be it "quote-tweets", be it text formatting in any way. However, all this stuff is perfectly normal and absolutely part of the culture everywhere outside of Mastodon.
1. Character limitation. It's funny that it is a reason because three are Mastodon-powered instances (a lot in fact) with more than 500.
2. What the "purists" did not realise is that the customizable number of characters has been part of the Fediverse since the first Fediverse managed service launched: StatusNet "hosting", in 2008. @-lnxwalt was very active in posting different content on the different length instances back then.
3. To add to what you mentioned, another reason they hate #Threads is because they supposedly introduced something that isn't part of #Mastodon and #ActivityPub, dots in username.
But it has been around years before Instagram even thought about Threads (assuming they came up with the project during the #TwitterMigration ). You'll often see it from website-based AP implementations. @example.com@example.com
Also, the reasoning "not in ActivityPub" is well, Mastodon is as guilty of that since the discussions of AP started. Since Mastodon was successful in positioning itself at the proper place, everyone ended up / was forced to implement Mastodon-only stuff just so their software will be interoperable with it, even though it isn't part of ActivityPub.
Most "Mastodon purists", as well as, "ActivityPub gatekeepers" use reasons that can't stand; and their solution is fragmentation or contrary to what the Fediverse is, and what AP was made for. Basically, they want to go back to the way things are: "siloed networks" or "walled-gardens".
#BringDownTheWalls has been the goal of the Fediverse since 2008, and even before that when #SocialWeb discussions started (even in related conferences).