A relatively small thing: the 500-comment viewing limit for normal accounts. So many times on Reddit I've been put off engaging with posts with 500+ comments knowing that nobody would see it. It's stupid because comments are just text and unless the software design is absolutely terrible then simple text comments shouldn't take up bandwidth at all.

Getting banned in one subreddit you never participated in for daring to have a comment (regardless of the content of that comment) in another subreddit.

Censorship. All the major subreddits became political echo-chambers. Reddit was founded on free speech and open discourse, especially when it was really uncomfortable. I'd love to see the same for Lemmy. Over the years I've seen authoritarianism creep into the moderation policies of most major subreddits. Today, even posting on the wrong subreddit is grounds for being banned from dozens of major subreddits. Even having a polite disagreement about, for example, anything to do with "trans," is grounds for being banned.

As a new community we need to identify and stamp out bad actors immediately and thoroughly (spammers, selfservers, ads disguised as posts, brigading, illegal content, racism, you get the idea).
We can't control if they create their own instances, but we can isolate them.