I have a couple:

"Posting the same content to multiple comms is a bad". Posting to multiple comms should be encouraged so long as it is relevant to each of them. The Threadiverse is federated and so there are many comms for the same or similar content, not every user is in or wants to join every single one. Your feed is yours and out should treat it as such, don't complain that another user is ruining you're feed because you decided to sort by new and are in multiple of the same or similar comms.

".ml are all just trolls / tankies / bad". The few trolls on .ml are given infinitely more weight than the genuine ML users, and the .world zionists and centre-right liberals are propped up as exemplary users despite constantly stoking a flame-war that most users don't give a shit about. ML has just as much of a right to ban political dissidents from their comms as any other instance. Explicitly leftist instances exist for a reason and you don't have to interact with them if you don't agree.

This last one is more of a subconscious view within the "Lemmy hive mind". The Lemmy defaultism gets exhausting (even if a minor grievance) and I wish "Lemmy" was reserved for things that are actually only relevant to Lemmy as opposed to using it as a general term for the threadiverse. I know that Lemmy was the first and is the largest and all, and it's understandable that it would be seen as the default (similar to Mastodon for the microblogging side of things), but I wish there was more of a culture around getting users to stop referring to the whole federation as if it's a single piece of software (similar to the culture on the microblogging side).