I'm a third through the corebook (after playing one game as a player), and I'm still surprised how much I love this. I was never much into the class-bound playing style, I always felt boxed in by it, but I must say that DH seems to embody the best of it. What it has going for it:
I can easily see how reflavouring would work for most classes - maybe it's just because I'm coming at this fresh, but I can imagine creating a psionic noir gumshoe as a bard here with little trouble. Part of it is relative freedom in choosing cards from your domains. It never felt like this in any versioin od D&D.
I really love the initiative system, which solves one of my biggest GMing problems (because I ALWAYS lose track of initiative). Just keep track of fear points, and let everything else happen as the players come up with it.
It's pretty well-written. I'm having fun reading it.
There' surely a lot more. There also some visible downsides; while it's great that there's no artificial restrictions on who can use which kind of weapon and armor, that also means that weapon and armor stats have next to nothing to do with any remnants of simulationist ideas about them and are obviously there purely for game balance (so everyone can just pick whether they want better damage thresholds or be better at Evasion). It feels a little off, but I think I can live with it, and it's probably a good trade off in favor of not restricting a player from flavouring their Wizard as plate-armor wearing battle-axe wielder. If I'm not misunderstanding, you can absolutely do that without breaking game balance (though it probably would be more effective to use some weapon that's not based on strength ...).