I really like Firefly. It had some really great characters in it and it's a shame it only lasted a single season with a film released later on, which tried to tie up some of the loose ends from the series. But I always thought its biggest problem was it was sci-fi show that didn't have any aliens in it. I think that most people if you ask them what makes a sci-fi series, a sci-fi series, it would be it has to have aliens in it.
A lack of Aliens was the only thing I didn't like about it.