They would introduce these new things, new ideas, new people, and these new things had their own great potential story lines, and yet they just weren't able to come together, and some of these new stories didn't go anywhere, some underwhelmed, some didn't materialize, and left twice as many questions.
It became clear that the only way to really make any of it work was for their to be another 10 seasons for every new story arc, but this wasn't possible.
I think looking back, they should have restarted after maybe the 3rd season with a clear ending in mind.