This doesnt apply only to sci-fi, but really to all television. You need to be able to relate to the characters, to their struggles, their hopes and their lives. Firefly did an excellent job creating a futuristic setting which is actually believable. The reavers are clearly the stuff of fiction, but a work can benefit from a certain degree of super-natural elements, so long as the core characters are grounded (which they are). Theres a limited amount of unfeasible technology compared to other sci-fi shows, and there are plenty of little touches that add realism. The characters speak a future dialect of English, with slang which is understandable while being new. They occasionally intersperse their conversation with bursts of Chinese, and having spent a lot of time as a native English speaker living in other countries, I can testify to the authenticity of this process when two languages collide. The fact that you can take the world seriously is invaluable. As much as I enjoyed Buffy and Angel, occasionally the shows seemed to parody themselves. The monsters lost their fear factor, and the characters even began making jokes about the improbability of what they were doing. Perhaps Firefly would have fallen into that trap had it continued, but as it stands it walked the tightrope between comedy and real danger perfectly.