Well, Firefly did instantly for me what a show like BSG took half a season to do. I think it's a sign of a well-written and well-done show. Firefly was a fluke. Cast got along. Story was good. All went well except for the ratings.
I can understand if people need more time to get into a show, but with a show like Carnivale (for example,) I was into it after 2 or 3 episodes and knew it was going to be one of my favorites. Sometimes, it's content that's appealing, sometimes it's pacing, writing, storytelling, acting, character arcs.
I am a writer myself and I've become the kind of person who usually knows if I'll like a book by the first chapter. Certain elements simply have to be there. I have rarely been surprised by my initial reactions. I think this has carried over into films and television shows, though with television the first few episodes can sometimes be awkward, bulky, clunky due to a cast/crew learning how to work with one another (provided they shoot in some semblance of order.) Firefly clicked immediately with me.
Carnivale clicked immediately with me but a lot of that had to do with content that was interesting to me (supernatural/good/evil/the occult.) It was also some excellent writing and story-telling/acting.
I also like the fact that some of these shows use actors that are not terribly well-known. I mean, you have Clancy Brown in Carnivale but he's more of a cult fan's actor (Highlander, all those prison guard roles, etc.) It's refreshing to see new faces.
I'm babbling.
I'm a nerd.