Looming doom generally hurts my enjoyment of a show
Here's something that I've not so much discovered as realized recently: I generally don't really enjoy a show where there's doom looming over the characters. This was a factor in my initial reaction to Symphogear and it's come up again this season in Mobile Suit Gundam - Iron-Blooded Orphans.
It's not that I'm opposed to character death (although certain forms of it turn me off) or that I require happy endings from my shows. I think it's perfectly okay to kill characters, even in casual ways, provided that it fits the show and the mood. What's different about a show with doom looming over things is exactly that there is doom looming over things. If I know some of the characters are going to die, I can't watch the show without wondering who it is and when it's going to happen. Is it going to be this episode, this next scene? When is the knife going to be jammed in and twisted? One of the common effects of this for me is to devalue much of the work the show is doing to develop characters. Actually caring about doomed characters feels somewhere between wasted effort and falling for the show's emotional manipulation, which irritates me. And when I don't know who's doomed when, well, pretty much all of the characters get affected.
So on the whole, a show with looming doom winds up being kind of a strain to watch. There is a constant tension and worry in the background that I don't like; it's simply wearying.
Despite this I think that shows can have looming doom and still work. While I don't have fully formed thoughts on how yet, my incomplete thoughts are that an important ingredient is for the show itself to acknowledge the looming doom by having it affect the shape of the story and the characters. In this I contrast Sidonia and Iron-Blooded Orphans. IBO is has relatively consistently ignored the doom looming over the cast, with essentially no sign of it in the story, while Sidonia embraced it in the atmosphere of the show and even the character reactions from relatively early. As a result this aspect of Sidonia worked for me and did not get me down, whereas watching IBO remains partly wearying and tense in an unpleasant way.
(An interesting question is whether I'd be enjoying IBO if I didn't know the outside-the-show information that Mari Okada (the show's writer) both quite likes melodrama and has said that there's going to be suffering in the show.)