I like to imagine it this way:

There's a party. You've heard of this party through your social interaction with other people. In the party, there's a lot of groups, all talking amongst themselves. You pretty much know how to behave in a party, through your social interaction with other people. Sure there might be few things to learn here and there, but overall, you know how to act. At least you'll pick up the important things quickly. Now, right when you walk in, you're faced with a group of friends who are talking to each other about dank memes. You don't know these people, or what the hell they're talking about. They seem to have a lot of in-jokes and weird terms for their shit. You get the overall picture, but you're lost when it comes to finer points. You can't really participate in the conversation, without either spending time listening to them talk or asking about what they're talking. Now, they don't mind explaining things to you, but people are constantly asking them and because they're constantly explaining what's happening, the conversation is less and less about dank memes and more about other matters.

Now, this group loves dank memes more than anything and they'd love to have more people to talk about the subject, but they sense that the conversation is turning too general. The subject is now something everyone can talk about, even though the original talking points were clearly defined. Though the conversation now is easier to approach and enjoyed by a larger crowd, the original commenters feel like this isn't what they wanted to talk about. Some of them decide to leave and talk about the original subject in someplace else.

A similar group decides not to stand too close to the door, in hopes that the same thing doesn't happen to them. They love to talk about the unexpected, but they don't feel like standing right next to the door is the smartest choice. The people that approach them have been in the party longer and picked up a thing or two about this party overall and about the group, so they know better how to interact with them. And some people have been pointed towards that group, so they have an idea what the group is talking about and actively want to take part in the conversation. Others have been told about them, but have decided to stay away because the subject doesn't interest them. The group isn't as big as the right next to the door, but the conversation is more on subject and the new conversationalist are better suited to talk about the topic. There's less explaining to do and the people who do approach them are more excited about the topic.

The social interaction is internet as a whole, the party is Reddit and the different groups with different topics represent different subreddits.

I feel like more general subjects are better suited to be defaults, since there's less to explain and to learn. And the subject matter is usually already something most people can enjoy. "Genres topic" subs are a great way to learn much of the in-jokes and the "etiquette" that makes Reddit Reddit. I know what I wrote can sound fucking pretentious, but my intention certainly wasn't to be like that. I guess I just kinda wanted to say what I said in this paragraph. All the other stuff was pretty useless, now that I think of it.

What a mess. I can't believe I wrote something as stupid as this shit. My God. Hope you skipped to the end instead of reading all that garbage I just came up with. Jesus. Why.