I wanted to open such a thread too. But then I decided to make the lists, so I postponed that. So I'm glad to read this now.
As HiveMindEmulator already pointed out, you have to differ between buying and actually playing the cards.
Regaring buying: This is maybe the main part in Dominion and also the most difficult one. So I may say: Each card needs skill to buy it as you build your strategy around it, even Silver or Gold, as you pointed out. Yes, there are cards that need more skill in buying, this are cards that need special strategies, like
1) Rushes: Victory cards (Gardens, Silk Road, Duke), Ill-Gotten-Gains, etc
2) Strategies that center on one card and you need special supporters like: Horn of Plenty, Hunting Party, Apothecary, Minion, Scrying Pool
3) Bad or mediocre cards, that are ocassional very powerful: Quarry, Coppersmith, Duchess, Wishing Well, Pirate Ship (just to name a few)
4) Potion cost cards in general
But: I like to hear really something about really playing the cards. As I made some thoughts before (this is going to be a longer post), here are my opinions:
I have several categories here too:
1) Cards with several options: This one is the most obvious one. Cards with options need skill to know which option to choose in which situation. Governor is probably the most difficult one as you have to not only take your hand into account, you have to take into account the hand of your opponents too. The others are (not necessarily in order): Steward (e.g. with a hand of Steward, Silver, Copper, Copper, Estate. Trash or $2 and Buy Gold?), Spice Merchant, Minion, Native Village, Pawn (when is +1 card and $1 worth it?), Jester, Mining Village (do I want to trash it yet?), Trusty Steed. Cards like Pirate Ship, Nobles, Spying Decision Cards (Navigator, Spy, Scrying Pool, Oracle), Loan, Chancellor also have decisions, but are most of the time easy to choose.
2) Trash-for-Benefit cards and pure trashers in some extent too. These include:
Apprentice (which card should I trash to be sure to get that Province and still be able to have enough money later too?),
Salvager (not as difficult as Apprentice, but similar, especially the option to trash a Province for a double Province buy)
Spice Merchant again (should I trash that Silver?)
Upgrade (especially because of that additional card and if you don't want to trash necessarily),
Remodel (do I need that Gold yet or should I remodel it into a Province?), Expand is similar
Develop (what cards are on the board that I would get?)
Forge (long calculations measuring the card to gain and the money left in your hand)
Remake is lesser extent
Transmute (do I really want another Transmute or a Duchy?)
Lookout (what 3 cards may I draw?)
Trader (do I want more Silver?)
Moneylender, Mine, Chapel, Steward (if I already chose the trash option), Jack of all Trades, Trading Post, Trade Route are easier, because they have either restrictions (e.g. only treasure/no treasure card to trash), are "trash-for-no-benefit" cards or have decisions which are easier to calculate.
3) Discard-for-Benefit cards
These are not so difficult to play, but can be. Especially if you have still actions left and will draw more cards. And this is especially difficult if you can profit from bad cards like victory cards with other cards you may draw, but still haven't in your hand yet (Crossroads, Baron, Tournament, etc).
Cellar (do I want to discard my Copper and hopefully draw better cards?)
Hamlet (do I need that +Buy yet?)
Cartographer (not discard-for-benefit per se, but has decisions like Cellar and fits in category 1 too)
Stables (do I want to discard that Silver?)
Tactician (this may the most obvious one)
All other cards that need to discard, you just have to take the order in which you play them into account, but most of the time it is late in your turn.
But if you have nearly no cards left to draw, this can difficult too, like playing Vault before drawing more cards to get them all back into your hand.
4) other cards which let you choose a card
Throne Room and King's Court: Which card do I want to double or triple play? This isn't always obvious!
Cards that have to set aside a card like Courtyard / Herbalist / Mandarin / Haven / Island
Scheme (Which card do I want to put back?)
Library (when you have an action left and can choose to discard which action card. Hopefully there will be coming better ones!!)
Masquerade (especially when you multiples in your deck, the order of giving your opponent a card I found always confusing)
Ambassador (is mostly easy, but Estate vs. Copper first is still a valid question)
Embargo (on which pile do I want to put that token on?)
5) Cards that give benefits that aren't always good. So you have to think about it to play them at all.
Bureaucrat, Explorer, Mint and Trader (do I want more Silver?)
Smugglers (wait, which card have you gained?)
Contraband (do I really need $3 more?)
Baron (do I want an Estate?)
Extra gains like:
Haggler (you know that you have to gain a Copper for that Hamlet?!)
Talisman (you have to gain a second copy!)
Horn of Plenty (do I want another $4 card? or: do I want to trash it yet?)
6) Cards that profit your opponent:
Govenor like already mentioned, Vault (especially in cursing games), Bishop (can be difficult on both sides), Council Room, Duchess