Lack of humor. I get that the show was going for gritty and dark. But BSG probably had the least amount of humor of almost any show I'd ever seen.
Consider putting yourself into this very apt analogy:
You're locked aboard a commercial airliner, say about the size of Colonial One. You have no real possibility of ever EVER getting off. You can't open the door and leave. You can't open a window for some fresh air. You can't bathe properly; a sponge bath is likely the best you'll get. Everything you eat is made from algae.
There is no real government anymore. There is no military for that matter. You and everyone else pretend that such things still exist because psychologically you must
. But the ugly truth is that the rules are being made by the people who have the guns, and most of the time they're making it up as they go. There are no heroes, not really, but in spite of it all there remain a small few who actually try to do the right thing--when they can.
And above all, you're being hunted. Relentlessly, with no hope of anyone coming to your rescue, because you and your fellows are completely alone,
and will be for the rest of your lives.
I think humor would be one of the first casualties.