Exile is an excellent mechanic, because you can do really simple things with it, with interesting results. Stockpile is a great example. A card that exiles itself on play, that's all it is, but people on Discord have been arguing about how good it is all day.
I can imagine a workshop variant that exiles cards from the supply. A card that blindly exiles cards from the top of your deck. An exile-for-benefit card. An attack that exiles opponent's good cards. I'm looking forward to seeing what else the set has.
Bounty Hunter - Why is this so strong? People initially underestimated Priest because they didn't understand how it trashes cards and helps you hit $5 at the same time, much like another power card, Masquerade. But bounty hunter gives more money, it is nonterminal, and it thins provinces without losing VP? Would it really have been too weak if it only had two of those three? I wouldn't be surprised if Bounty Hunter makes you green much earlier in most games.
Stockpile - Definitely one of the harder cards to wrap your head around. People on Discord seem to think it's good to pile these, but I'm not so sure. Surely sometimes you can do better than having a bunch of gold in your deck. The thing I like about this card, is imagining what happens when it's the only +Buy. You have to buy several of these over multiple shuffles before you net much +buy out of them. And if they pile out, that's it for your buys.
Coven - I think this is usually easier to deal with curses one at a time. It gives you time to get control of your deck, and when the curses arrive you're ready for them. But I really like this card. It makes you rethink what kind of trashers are good or bad at responding to it. And it gives your games a narrative arc.