I agree that it's a nice card when there are no Attacks on the board. When there are attacks, but ones that noone wants or that noone wants lots of then it is essentially a weak moat (a once-per-turn moat).
As I mentioned, it's great against minion, and in a very interesting way. Usually you either have to race for the minions or you let your opponent have free reign of the pile. Peacemaker allows you to virtually compete in the minion race while not committing your deck or buys to being a minion deck, thereby allowing for interesting strategic differences. And as defense, since you are setting the peacemaker aside you will have four card hand which is immune to subsequent minions. And all this without totally shutting down your opponent's strategy, so opponents will still sometimes opt to go Minion. All in all A+ design for minion games.
It's also great fun vs. Cultists. Cultists are another card which is often acquire en masse and chained together. Peacemakers a) prevent you receiving a Ruins, b) virtually participates in the Cultist rush in the same sense as the prior paragraph, and c) allows you to draw 3 cards since a Cultist is trashed. Your opponent may draw an additional Cultist to play, but if you bought lots of Peacemakers you're just as likely to have drawn another Peacemaker. You may have even more "Draw-splosion" for the Cultist chain than the player performing the Cultist chain. Sounds really neat.
But any game with attacks, but not ones that you want to purchase en masse (e.g. Minions, Cultists, Torturers, Scrying Pool), it doesn't seem like the trashing will be all that significant, and it seems unlikely that attack-free Peacemaker will be activated. It's even less likely to activate if there are two or more attack piles. Moat can be useful as soon as there's a nasty attack (particularly in 3P and 4P games). Put Witch in the game, and Moat can be useful. Put in more attacks? Say Mountebank, Militia, and Cutpurse? Moat is only more useful. Yet these additional piles make it even less likely that Peacekeeper will activate and become more than a Moat. Now, I suppose that's fine. It's awesome when no attacks around, a once-per-turn Moat when attacks are around, and extremely interesting when there are Minions or Cultists (I haven't looked through the card list, so maybe another nice match). But the thing is, the cool mechanic here is the trashing and the activation once no more attacks are left to trash (kind of like City!). And that's a cool mechanic. I just feel like it's going to be so rare that this mechanic impacts the game. I don't mean to dismiss the card as a concept, I'm just wondering how the card could be tweaked so that this mechanic could really have some impact outside of a few niches.
