Discovery: I think there are some potential logistical issues with the card, but I understand the intent. Gain two copies of a card, one top-decked, but virtually gives every other play a copy of the card in hand.
The wording is a bit confusing. "each other player may play a card..." so it doesn't cost them their action? That's very powerful for them. But then, Discovery has no cost restriction, which is kind of insane. Gain two Goons on t3? Gain two KCs later? That's powerful enough that I'm not sure the huge benefit to others matches.
There's also a lot of potential confusion to be had when people are playing Estates as action cards.
I think a cost restriction would help a lot. As is, it sounds broken... but it's unique enough that it's hard to say.
Printing Press: So, major sifting, copy and then bottom-deck. If you can chain two of these together, you can select which card to "print". Interesting concept. I don't think it is too powerful. Despite the heavy sifting, this card can't increase your hand size. It actually decreases hand size if you choose to bottom-deck. The "printing" power is tempered by looking at the bottom card, which is much more likely to miss the shuffle anyway. Also, the first play of it can whiff fairly often except in already well-crafted decks.
I don't think it's broken, and it's worth testing as is. If it's too powerful, you could have it discard 6 cards, or just lower the draw (and maybe the discard as well) so it has less reach. Or maybe make the bottom-deck mandatory. If it turns out to be weak, I think it might be worth leaving it as is anyway.
(PPE: yeah it could be pretty ridiculous with KC, but I don't see it as being too much crazier than other possible KC-KC combos).
ThinkerSo this is huge non-terminal draw that lets your opponent semi-Island one of your cards. Since there is a way to get cards back though, I'm not sure it's enough. With some +Buy and a decent $2 card (or Watchtower), the penalty is easy to overcome. Heck, with such huge draw, it could be OK to just buy Copper to get your cards back. I think it might be interesting if the removed cards were permanently gone (until the end of the game).
I may be underestimating the severity of the penalty, since it
does prevent you from playing the taboo card at least until your next turn (excepting Black Market). I think it really does have some game-breaking potential though.