Dark humour, often then not most grimdark book fail at this and we end up with juvenile American humor instead. The humour should be on Joe Ambercrombie levels.
I'm going to be talking about it at the end of this week on an online convention panel (with some incredible authors)
Nihilism and sarcasm. For me, grimdark has to a) show the futility of existence, and b) do it with a wink to show the author knows they're going over the top. A lot of modern 'grimdark' has abandoned that humour. When I look at Godblind or We Are the Dead, the lack of humour is a massive turn-off. It's not my only issue with either book, but it's the biggest.