Babylon 5, Deep Space 9, Farscape, Whedon's shows, they all got at least close to as Dark as NuBSG. Current shows Falling Skies and The Walking Dead (I don't know the Pay Cable Fantasy Shows) also get just as dark. But, they aren't non-stop bleak, they break up the dark with jokes and good times, so, although you may go on an emotional roller coaster, you have feel good moments too. Hell, even Once upon a Time, is much darker then it appears at first glance, and that's on ABC(Disney). So, I don't think dark is a problem, balancing that drama/sadness with hope and humor and good times, keeps them fun, despite the darkness.
So, as long as they don't dwell in the pits of despair or crack inappropriate jokes at the wrong dramatic moments, darkness isn't a problem for me, and I think in today's TVscape, you do need some.
Someone mentioned above the problem with screwing over likable characters over and over again, it's just as deadly not to have enjoyable characters for fans to latch onto (Enjoyable characters don't always have to be heroes, they can be villains [Dukhat, Kai Winn, Jeffrey Combs' characters])