Here is what makes this series great first of all: the cast.

From left to right: the tough-muscle guy that wants to earn as much money as possible; the weird and mystic young sister; her doctor brother; the captain; the "companion"; the tough fighter/wife; her pilot husband; the mechanic (yeah, that's the girl!; and the sheperd (yes, a shepherd!).
These 9 people are all onboard Serenity, a Firefly-type carrier spaceship in a solar system with many planets and moons. The inner planets, are luxurious, nice and warm. In the outer planets the alliance is not so strong; the people are poorer and the facilities not so good.
On top of the great and ultra-smart scenarios, and the cowboy-like way the people talk, there are a few details that make this series special for me. First, there are no sound effects in space. No air, hence no sound waves. Second, they sometimes speak chinese instead of english. That is because it depicts a future where USA and China are the 2 superpowers that colonized space, and now the remrants of their languages remain. Third, there are no aliens! I always found them annoying. In Firefly there are just humans, plain old human interactions.
The writer Josh Wheedon said that he will continue the mythology if the movie goes well. The dvd of teh movie comes out this week, so I hope it will all go well...