I always thought there was no big difference between link aggregators and forums, and that the first is more an extension of the other, at least it felt like it in my experience. What would you say really sets them apart?

This thread is a great example of the big downside of being on a smaller instance: OP was seemingly unaware and unable to subscribe to many cool communities and was stuck with the “defaults”.

Perhaps admins and mods of smaller instances could be doing more to educate their users about how to find stuff in the wider Threadiverse?

When we had the big migration last year everyone quickly learned how to search for a community, how you had to do it twice for it to show up etc. But now it’s no longer a common topic of conversation so newer users don’t realise, and to make matters worse every single mobile app seems to handle search differently.

Is there some kind of “so, you’re on a small instance, here’s what that means” type of resource already hanging around somewhere?

(OP, get yourself on lemmyverse.net/communities)

Mastodon is much more this way than Lemmy, in my experience. I honestly don’t know too many Lemmy communities that are that way, but kbin / mbin will let you interact with Mastodon users on a chat basis in addition to Lemmy communities.

tildes.net might be what you are looking for.