As mentioned, the Intrigue expansion was Dominion’s first chance to vary the environment and give more potential to the less valuable cards. Remember, the benchmark is Big Money and streamlining decks, so the design space to be explored is how to make the cards that are not the best still worth seeking. Silver, Gold, and Province are good cards that everyone wants. How do we make Copper, Estate, and Duchy more valuable?
Baron makes money off Estates and gets you more Estates. That’s good. That gives you victory points in small increments while helping you buy them in larger increments. And remember, at the end, you only need to win by one point. Baron also helps you get a quick start, because your odds of drawing Baron + Estate on turns 3 or 4 are pretty good, but if you don’t hit that, your turns will be horrible because you have one turn with Baron and no Estates then a turn with Estates and no Baron.
Duke is worth more victory points when you have more Duchies. That is a straightforward way to make Duchies more appealing: synergy! Duke is kind of the opposite of Baron: Baron shoots for the extreme turn with lots of money (or misses for nothing); Duke works reliably at $5, and you just need cards to support it that let you reliably get moderate amounts of money.
Coppersmith makes Copper produce an extra coin this turn. Straightforward: Coppers are now effectively Silver. This becomes more valuable with multiple actions or cards that double or triple actions. Coppers that are worth as much as gold are not only valuable, but you did not spend the money buying gold.
But you did spend the money buying a Coppersmith or a Baron or a Duke. I do not find this expansion very successful and offsetting the base value of Silver, Gold, and Province. When Baron works, you get a nice head start, but that’s a bit of a gamble, and Baron only gets worse as either you have too few Estates to fuel Baron or too many to do much when you do not draw a Baron. In a long game, a Duke deck can beat someone playing for Provinces, but you are relying on the game running long enough for that investment to pay off without your opponents’ investments paying off sufficiently. Duke decks also get weaker as more players play them; being the only one playing for Duke is a great position to be in, but being one of two or three means the other player(s) get ahead while the dukes squabble. And Coppersmith is just a weak card in most situations. Yes, it is great if you can play two and turn your Copper into Gold, but you need one card to get you +2 Actions plus your two Coppersmiths and then how many cards do you have left for Copper? If you have that many cards and actions, there are probably better things you could be doing than playing with Copper. The opportunity cost of a Baron or Coppersmith is not buying a better card, and then your deck has more Estates and Coppers than you want except for the turn when your combo pays off big.
But future expansions will come back, try again, and in some cases make cards that combo wonderfully with these, perhaps adding enough value.