The Huffington Post brings us the following:
Will there be a Big Bad for the season? Is that something we'll see on this show: a villain having a big arc that grows, or will it be more mini-arcs or standalone episodes?
(Maurissa) Tancharoen: I don't think we can't be too specific about that but yes, we will have --
(Joss) Whedon: Recurring elements.
Tancharoen: Yes. There will be threats. How vague can we be? [laughs]
Whedon: Yeah, one of the things that, you know, we've talked about just initially getting into it is, as you start it, you want to tell these self-contained stories. It's a little bit like the "X-Files" model: You can come each week and see [that episode] and not have seen everything, but if you have, it's a richer experience. And as we move forward, those things will start to weave together more and more. But we do want to always have every episode have its own beginning, middle and end, and feel like its own [thing].
Well, "X-Files" or "Lost" would also have straight-up, hardcore mythology episodes, too. There would be, like, five monsters-of-the-week episodes and then in the sixth, it would be much more about the bigger mythology.
Tancharoen: I'd say it's similar to "X-Files."
Whedon: Yeah, we definitely don't want anybody ever to watch an episode and have to have watched all the others. That's definitely a goal of ours. But we think there's a way to do that, [similarly to] the way that they've done it with [Marvel films] where it's just a better experience if you have been keeping track. And also we're pretty sure that we will be asking enough interesting questions that people will want to hear the answers. At least that's our goal.