I'm amazed this thread made it 48 posts before anyone mentioned Buffy.
For folks who don't know, Firefly was created by Joss Whedon, the creator of Buffy and Angel. Folks who like Firefly would be well-advised to check those series out -- the dark comic timing mentioned upthread is present there as well.
Ya know, I just didn't think Whedon was "all that" before Firefly. I got maybe three episodes of Space: Above and Beyond before the high cheese outweighed the interesting 'world' for me...a pattern that continued through his more famous Buffy and Angel series.
Now Firefly is a different breed of cat. The only thing I've seen in the entire season that strained credibility past the breaking point was Mal letting Nizka live.