Curses vs. Ruins.
Everyone who has played the base set of Dominion knows that Curses are bad cards. They are good to screw up your oppenent's deck and they also give negative points. Someone will probably like attacks that give Curses and they will try to get rid of these awful purple cards as fast as possible when they happen to be in their deck.
Moving through the expansions, someone meets new awsome Cursers and new ways to get rid of Curses, or live with them. But when they reach Dark Ages, they will have a pause on Ruins. They will wonder. What are the Ruins? They will experiment with them. And after getting the experience, they will see that Ruins are pretty similar to Curses: Their purpose is to screw up someone else's deck, but they have some obvious differences with Curses: While Curses are totally useless cards and give negative points, Ruins don't give any negative points and they are actually Action cards that can be played and even give you some minor benefit.
Looking through the games being played in Goko, most games include random cards from various sets and that means that the Ruins may sometimes inflitrate a game where you have gotten used to seeing Curses instead. And this way, you start noticing that some cards interact with Ruins in a different way than they would interact with Curses.
There are a bunch of cards that behave differently with Curses and differently with Ruins and some players are probably not aware of that, simple because they haven't noticed or haven't thought of it yet.
I am now going to display a full list of these cards, because i believe that it's good for everyone to keep in mind these various interactions and have knowledge of them before going into a game, because some strategies can be totally altered by the presence of Ruins (instead of Curses, maybe).
Curses: It's draw can be triggered with Curses in hand.
Ruins: It's draw cannot be triggered with Ruins in hand.
Curses: Cannot get benefit by hitting Curses.
Ruins: Can get up to +4 Actions by hitting Ruins.
Curses: Will gain Curses while not giving any benefit.
Ruins: Will gain Ruins while giving +1 Action.
(That's probably useless unless you 're playing with Vineyards or trying to 3-pile, but i had to point it out).
Curses: It will skip them.
Ruins: It will stop on them and play them.
Curses: It will trash them into nothing.
Ruins: It will trash them into Duchies.
Curses: It will stop drawing upon revealing a Curse.
Ruins: It will not stop drawing upon revealing a Ruins.
Curses: No interaction.
Ruins: They will make Vineyards go up in VPs.
Curses: No interaction.
Ruins: It can gain Ruins.
Curses: It will skip the Curses.
Ruins: It will not skip the Ruins.
Curses: It will top-deck the Curses.
Ruins: It will not top-deck the Ruins.
Curses: Will give +$1 by hitting 2+ Curses.
Ruins: Can give more than +$1 by hitting 2+ Ruins.
Curses: It's draw cannot be triggered with 2+ Curses in hand.
Ruins: It's draw may be triggered with 2+ Ruins in hand.
Curses: Having a Curse in hand will cause other Curses to be skipped.
Ruins: Having a Ruins in hand will probably not cause other Ruins to be skipped.
Curses: No interaction.
Ruins: They can be played and act as additional unique cards in play.
Curses: Curses count as 1 additional unique card in deck.
Ruins: Ruins can count as up to 5 additional unique cards in deck.
Curses: Revealing a Curse will cause another Curse to be gained.
Ruins: Revealing a Ruins will probably not cause another Ruins to be gained.
Curses: No interaction.
Ruins: Can shuffle into deck Ruins when you gain it.
Curses: No interaction.
Ruins: Can top-deck Ruins.
Curses: No interaction.
Ruins: They can count as additional Action cards in play.
Curses: It will leave Curses on top of deck.
Ruins: It will discard the Ruins.
Curses: It will give a Curse to each other player when revealed.
Ruins: It will probably give Ruins only to the player to your left when revealed, and only if you return it to the pile.
Curses: It will discard them.
Ruins: It will leave them on top of your deck.
Curses: It will not give any benefit by revealing a Curse.
Ruins: It will give +1 Action by revealing a Ruins.
Curses: No interaction.
Ruins: It can target Ruins.
(An obvious one, here.)
Curses: No interaction.
Ruins: It can trash them into a card costing up to $3.
Curses: No interaction
Ruins: It can target ruins and trash them (if Poor House is not in the game).
Curses: No interaction.
Ruins: It will play them upon revealing them.
[Edit:]
Curses: No interaction.
Ruins: It can skip the Ruins as it draws them and discard them after drawing.
Curses: By naming Curse, it can trash more than 1 Curse.
Ruins: By naming a Ruins, it will probably not trash more than 1 Ruins.
Curses: By naming Curse, it can skip more than 1 Curse.
Ruins: By naming a Ruins, it will probably not skip more than 1 Ruins.
[End Edit]
Now, i'm not exactly pointing whether the outcome is good or bad in each case, but i'm pretty sure you can all understand which one it is. For example, Golem and Hunting Party stopping on Ruins is definitely a bad thing, as is Shanty Town not triggering it's draw. Menagerie being able to trigger its draw with many Ruins in hand is definitely a good thing, as is for Fairgrounds to have more unique cards available. It seems that Scrying Pool can live with Ruins, while Curses stop it dead, and it could be interesting to target Ruins with Transmute or Graverobber. A Herald deck is in a somewhat better situation with Ruins than it would be with Curses and, with Procession, 99% of the times you will want to target Ruins particularly to trash them, but who knows? Someday, someone may even kick off their engine by processing a Ruined Village!
Now there might be a few more minor interactions that i haven't thought of yet, but i'm pretty sure that I have covered the most important parts of this aspect. If i'm missing something i'll be happy to be informed.
[Edit:]
How could i forget about these three cards? Especially Library! Anyway, thanks for the feedback. I just added the cards now.
[End Edit]