Kay Reindl at Seriocity has some really interesting things to say about pitching shows. She talks about how pitching works in LA and what some of her own meetings have been like. She goes on to talk about the components of the pitch, which are of course, the components of the series.

It's a great piece and you should read all of it, but here are a couple of excerpts. Reindl on the premise:

this should be clean and easy to grok. You shouldn't have to spend more than thirty seconds pitching your premise. And the rest of your pitch should only expand on that premise.

The characters:

they must rock. As simple as that. They need to make sense for the premise. An example would be the X-Files. Mulder's the believer. Scully's the skeptic. She's sent to keep an eye on him, but winds up having his back. Simple, right? Iconic, even. And they fit the premise. Your characters should feel like individuals. You should know their backstories, what got them into this predicament, why the show is about THEM, and how they react to what's happening to them. Arcs for characters are an excellent idea. Tell the executives where you want to take these characters. Know their voices, and incorporate them into the pitch.

Reindl on the engine:

this will be referred to as the franchise. What that really means is, what happens every week?

And on the world:

this is especially important when you're pitching a show that doesn't take place in our mundane, everyday existence. Pitching a genre show? KNOW YOUR WORLD. And the rules for your show. Does your show have a visual style? Know what that is.

These same elements are at the core of a great pilot script. The more you know about them, the better your pilot and your series will ultimately be.