Well, my brother was the one who got me into Firefly, and he told me I had to power through the two-hour pilot. He said it was good, but that it was needed to understand things later on and that the series really takes off a few episodes later.
I'd also go with the strength of the characters. Firefly has AMAZING characters. For a show that only lasted one season, the fact that they were able to develop them that much and make just about the entire cast that strong is a testament to the show.
The most important thing though, is like someone above me said, I would just say: "you have to trust me". The notion of space/cowboys/pirates/Robin Hood can be a little daunting for people who haven't been into Joss shows to wrap their heads around. And really, at face value if you've never seen it, it sounds kind of dumb. Once you get into it though, I haven't met many people who aren't Firefly fans.
