It was mostly the sheer amount of death and the apparent hopelessness of the battle. Taylor’s trouble with finding a way to help certainly didn’t help, either.
I’m okay with the named character deaths, including Tattle’s (though I’m not happy that my favorite Undersider died, I was sort of expecting one of them to – besides, I don’t really count her death towards 8.3, since we saw it happen in Interlude 8), but the way so many people died without making much headway against Leviathan at all, and the way the chapter essentially ended with “fuck this, we can’t win, let’s focus on surviving”… it was, quite simply, a chapter designed to tear down every shred of hope in the reader.
This is not a criticism. This is a situation that is very clearly supposed to be bleak, and Wildbow pulls that off quite effectively. Bleakness in itself isn’t a bad thing, it’s just a mood that I don’t always enjoy in my fiction as a matter of personal preference. (I sometimes do, though – you may note that I quite like Madoka Magica, for example.)
I think what will determine whether I like this Arc in retrospect will be whether or not Wildbow is going somewhere with this bleakness. Will the bleakness be employed effectively to emphasize the victory? Is it meant to convey a message that isn’t just “the world sucks”? Will we get to see Taylor deal with having been through this experience, and the death of Tattletale? It seems that what will make or break this Arc is what happens at, and after, the end of it.