Yea, the complete lack of humor to break the tension is what I too figured out weighs it down so much.
As far realism, that's my one complaint, is that they went so far in making the show gritty and realistic and avoiding any kind of joy, it became unrealistic because even here on Earth in Wartime, while your fellow soldiers are getting killed around you, the Soldiers still take time out to unwind, enjoy themselves and just have fun, no matter how bad things get.
I've just watched Charlie Jade
, which gets every bit as dark, gritty and graphic as NuBSG does, and I think one arc in particular, goes even farther than NuBSG did, and yet, it doesn't feel as weighed down with doom and gloom, because there is one character, who while doing awful things, brings a little levity.
I did enjoy NuBSG, and do own it, and will one day get around to rewatching it, and I wouldn't even mind another Series coming out in the same tone, but, it would be very disappointing if that tone dominated the landscape, and there was no other degree of SciFi available for watching.