That's a complicated question, one I'm hesitant to answer due to the fact that my profiles there aren't hardened the way this one is.What would be a higher end hub to you? Legitimately curious.
For me, higher end hubs are focused less on things like fanfiction, or individual fandoms, and more on writing in general. They tend to be professional author heavy, both in people who are published, and people who are hoping to publish.
The biggest thing about them is that there is a skill barrier to entry. Not in that they discriminate, but in that the concepts they talk about are incomprehensible to anyone who doesn't understand the basics at a bare minimum because these higher level concepts are discussions about the more esoteric areas of writing. Things where there is no right answer, only answers on individual story by story basis.
Simply put, it's a high level hub if a new writer can walk in on a random but typical conversation, and understand nothing that was said.
On a low level hub, you will get questions like "How do I make a sentence?"
On a high level hub, you will get "Pantsers versus Plotters; the tendency for the gardener style of thought to produce four times as many plot shifts compared to Plotter story of the same length, and the implications this has in writing."