World was already the biggest by far when I first started lurking back in July, and it’s just getting more dominant. Before, there was quite some diversity in the distribution of generic communities, but nowadays the vast majority of posts that reach the top are from over there.
I really can’t see any specific virtue that it has; uptime is not the best (or so I’ve heard), the moderation is quite lacking (which is demonstrated by the fact that Beehaw defederated them), they make some unpopular moderation choices (like blocking !piracy@lemmy.dbzer0.com), and overall the atmosphere is a lot less… nice than those of smaller instances.
I also feel like it goes against the idea of the Fediverse that one instance has control over most of the platform. Especially on Lemmy, where communities mean that building community within an instance makes so much more sense than elsewhere, and upvotes are federated near perfectly regardless the size of your instance, decentralisation makes a lot of sense. It really just doesn’t make sense to me that Lemmy World is where people are going.
It makes a difference for the person hosting an instance. Suppose you’re hosting an instance with ten users, and you run into some kind of configuration issue, and stuff isn’t working right. Or maybe the server cost is more than you expected. You might just decide to let it shut down. If you have ten thousand users you might decide to stick it out because people are counting on you. Or you’re getting donations from a hundred people, so you decide to make it work because so many people are counting on you, or maybe there’s a specialist who’s also a user, and they help you figure out the issue.