Any card that offers a multiple choice takes skill.
Even Pawn with its six possible choices. A lot of players will automatically play it for +1 Card / +1 Action out of fear that they're drawing a dead action card. If you're paying attention to your deck, a lot of the time you can choose +1 Card / +$1 or +$1 / +1 Buy or even +1 Card / +1 Buy if you know it's likely a Silver or Gold.
Steward offers just 3 choices, but a good player knows when to choose what. I think I trash too rigorously with Steward.
There are some cards that aren't that hard to play once you have them, but you need to make a plan upfront of what to do with them: Apprentice, Bishop, Horn of Plenty and Silk Roads spring to mind. Also Mint, since you need a good reason to get stuck with your 3 Estates and just a few spare Treasures.
Contraband is a tricky card, since it requires skill from the other player. It's usually played first and then you have to think: Does he have enough for a Gold, a Platinum even? Do I veto Colony? Province?
Heavy Embargoing can make some games very weird very fast. While it doesn't take skill to Embargo something you have and your opponent wants, it takes some funky planning to plow your way to victory through an Embargo-ridden minefield.