I think I've figured out this season now. This idea has been mulling around in my head since the second or third episode, but it's really become clearer now. It has had a different feel from any other season. Every other season drives forward with a sense of purpose. A heroic (or villainous) sense of purpose. This season, or Volume as they like to name them, is named "Redemption." In order to devote an entire season to this theme, you need to establish that your characters are lost, broken and defeated before you can bring them back and redeem them. And in Heroes, "Shadowboxing," this begins to happen.
It took a while, and maybe this is part of the downside of this season, but after seven episodes of showing heroes that are just meandering and wandering around we're finally directing them back onto a redemptive path. Emma is realizing her medical potential, Claire is figuring out who she is, Mr. Bennett is back in the business of helping heroes and Matt Parkman is facing his destiny.
In this episode we also get somewhat of a glimpse into what the carnival represents. It's a sort of refuge. A place where people with abilities can go to exist without hiding. But if this is true, then how does this fit with a story of redemption? We know Sullivan has mistakes in his past that he wants Hiro to fix. That's redemption to be sure. But why is every hero being drawn there? The thing about a refuge is that it's often the only place you can find your true path in life.
There's not much more I can say without getting into spoilers.