Surprised everyone is arguing about bandit instead of mentioning shit like wharf, margrave, etc that are just beyond broken.
Broken or weak cards are explicitly not what this thread is about.
There are many examples of (arguably) suboptimal design in Dominion, most of which have been discussed ad nauseam (Possession, Rebuild, Governor, Swindler, Cultist, Spy and many other slow topdeck attacks and / or filterers). Other than those, I think the card design in Dominion is generally quite good. There are, however, some mechanics and some big picture stuff I dislike:
- Boons, Hexes and Loot; needlessly complex and swingy. Boons and Hexes were pretty unpopular already, why did there need to be Loot? In my opinion, Loot is actually even worse. The most meaningful difference between the Loots is often whether they have +Buy or not. Bad luck in that regard can easily decide a game, depending on how quickly you can get more of them. Apart from that, does the difference between, say, Amphora and Jewels, really justify a randomized 30 card pile you have to carry, get out of the box, shuffle and get familiar with?
- Potion costs (as mentioned by Gherald)
- The explosion of ways to play Action cards in the Buy phase, Treasure cards in the Action phase and Reaction cards just whenever you goddamn please is probably justfied by the amount of design space it uncovers, but there is a real cost with how much it distorts the flow of the game and confuses new players.
- The push of Engines has made Money and Slog games much less common, which is great for many expert players, but not so good for most casual players who don't typically enjoy sitting through 10 minute turns by their opponents. While Slogs and Money usually take more turns, those turns are much shorter and the games are more engaging and manageable. Also, it's now quite rare now to seriously have to decide between playing Engine or Money; Engine is almost always correct if there is a way to draw at all (you may not always have +Buy, but there are now so many gainers that single gain games are borderline exotic).