First I would want to cover the difference between horror and terror as I understand them. Terror is a visceral response, a reflexive action tied to the fight or flight mechanism. Gore, violence, and other traumatic events trigger the terror response. Horror is cerebral, and intellectual response that comes from the inability to change or accept something that is going to happen.
Terror is the zombie surging at you, teeth snapping and tearing at your flesh. Horror is sitting afterwards looking at the bite on your arm and knowing that in a few hours you are going to die a painful and horrible death and will become a mindless monster very likely to attack the people you care about, and the only way around that is to commit suicide in a fashion sufficient enough to prevent your reanimation.
Smallness and Indifference
Despite the great accomplishments of the Cosmic Era, the Earth is a ghost of what it once was, and there are reminders all around. Empty cities, crumbling structures built to house billions now reduced to millions. All the epic level characters written up won't matter in the face of the enemies of the Era. Star Whales would pass through the massive and powerful Federation warships, stripping them of their crews, leaving the ships filled with only corpses. A strain of teratomorphic influenza, a dimensional fatigue event, the appearance of a cosmic horror, and it's all gone. The heroes who stand against the monsters aren't going to win. At best they can force a draw. The Liu Kangs, Ryus, General Dukes, and sundry anime characters aren't going to win. They are likely going to die, go insane, become corrupted, or otherwise fall or fail.
I agree, I think this is the point I have made the strongest, and it is the one I think I am the most familiar with. It is also drawing more strongly from my personal experiences, and observations on technology and culture. In my opinion, this is also tapping into my own personal projections and thoughts on what has happened to us as a society and culture.
Bradbury is a writer that I haven't drawn a lot of lines to, Fahrenheit 451 would fit solidly into my vision of the Cosmic Era, but not so much all the Mars stuff. I like the terms vicious ignorance, and it fits. Despite the advances in tech, the general population is dumb, complacent, and accepting. That being said, compared to our previous generations, while we may act and feel smarter, we are generally less capable than they were. As the tech grows more sophisticated, the number of people who understands it dwindles. As it becomes more ubiquitous, the more people become addicted to it and complacent to how it manipulates their lives, until we reach a point where we struggle to remember basic passwords and only retain the most vital of phone numbers when I can recall having quite a few phone numbers once memorized.
The cult of ignorance, believing celebrities over professionals and experts, etc
Arcanotech is a disaster, it rips holes in reality, releases anomalies, diseases, distortions, monsters, aliens, and other horrific stuff. After Chernobyl and Three Mile Island, there was a moratorium placed on nuclear power plants and we still haven't moved past it. Tens to hundreds of thousands of people have been killed, maimed, mutated, or otherwise disfigured by failures of this technology, the Cosmic Era is insane for still using it.
DFEs are lethal, and the majority of reflex teams and first responders who arrive at one are among the first to suffer it's effects.
The Cosmic Era is brutally pragmatic. It sacrifices population centers to keep it's arcanotech infrastructure running. Clones are created for exploitation and experimentation in a manner that would leave the Tuskeegee researchers sick to their stomachs, on par with the acts perpetuated by the Nazis or the Japanese medical researchers. Dissidents are nerve stapled, the media is a leash on public opinion, and so forth. I think this ties in with Huxley's dehumanization, as I see a lot of this going on now, with media spin, and data mining.
Where's the HORROR
The horror is on the other side of the DM's screen. The players, the protagonists should build their kick ass characters, the sort that are broken out of the CE mold and are aware of the vicious stupidity, the conspiracies and manipulations, the scale and vastness of the setting, and their ultimate unimportance to it, and then try to be big d**n heroes.
The DM/storyteller then drives them insane (followed by psychotherapy, asylums, and medication) maims them (and replaces their broken bodies with cyborg parts, or grafted body parts, and more body horror) kills them (resurrecting them as clones, or robot copies) and eventually, they might succeed.
They close the DFE, but they haven't and cannot address the underlying cause, they cannot vanquish arcanotech anymore than the Amish can vanquish electricity. They defeated the conspiracy, but only a single layer of it, there are more.