When I was talking to my wife (@tokolovesme), who still primarily uses X, about this topic, she said that she feels isolated whenever she uses Mastodon. In particular, when you're on a low-population server, you can't easily search for other people's posts, you can't see all the replies to the posts that flow in, and you actually see far fewer favorites and reposts. All of this makes the fediverse feel less lively.