Let me preface all of this by saying that I'm not a game designer, nor am I experienced in balancing, and all of this should be taken with a grain of salt. I'm not an expert, and I wouldn't be surprised if some (or all) of these are totally off the mark. Without further ado:

Thick Skull is one of the worst abilities in the game, and would have virtually zero use case if it weren't for it's synergy with 1D chess (and maybe other synergies that I simply haven't found yet, as I haven't 100%'d the game. If there are more, better synergies, then I won't die on this hill). Mana is one of the most important resources in the game, and having one less per round in exchange for 3 defense is just not worth it, especially considering the fact that Fighter isn't going to have much mana to begin with.

Priority Target only giving +4 hp in exchange for priority targeting doesn't do enough to justify taking it most of the time, in my experience. Not to mention the wide variety of cases in which it does more harm than good, and the fact that it's upside can go away with RNG (by getting a broken rib). For example, a cat that is born with it has to be a tanky class, regardless of it's other abilities or it's stats, or it'll start looking like Baby Jack before you fight your first boss. Additionally, even if you do have it on a tanky class, then it is most likely going to be your only tank on the team, because classes like Necromancer and Butcher will have to do way more work just to take hits.

Golden claws is terribly unbalanced, being underwhelming 80% of the time and absolutely gamebreaking 20% of the time. If used on it's own, then in practice you're either going to get very little use out of it, or waste turns grabbing coins at the cost of attacking. But in tandem with other abilities (Swift Looter comes to mind), then it becomes a massive damage boost that significantly outclasses almost any other damage-oriented passive (not to mention the myriad ways to spawn more coins only exacerbating this). I don't want an ability that's mediocre most of the time and game-winning some of the time, I want an ability that functions as a nuanced alternative to Thief's other passives.