^ Well, I think that B5 had it's own darkness - Down-below, Zahadum and the whole concept of the Shadows, the grimness of what the Centauri did to the Narn, the fall of Centauri Prime and Londo's fate, etc.
But then that was contrasted against more hopeful and 'light' themes and places. I always loved the look of Minbar, for example - it looked so....bright, serene, and mystical. And there were some very positive characters on that show...and positive events. Not everything was grim and a huge struggle.
I quite like BSG - it's probably my favorite of the scifi shows currently airing new episodes. But it can get overwhelming sometimes. Even for a DS9/Ron Moore fan such as myself.
Sometimes you want to just say: Can SOMETHING good happen to these characters, please?