View from level 39: I am strongest when I completely flout all of the conventional wisdom of the game. I am strongest when I refuse to buy Province, or buy weird cards that everyone hates, or use Throne-roomed Feasts as a makeshift source of +buy on a Highway set.
Here are two important pieces of anti-conventional wisdom:
1. The game length is extremely variable.
I know that sometimes, Province is a trap card. I love watching people build for a 14-turn game and realize that I've turned it into a 29-turn game. I buy Provinces significantly less often than my opponents, despite winning 2/3 of my games. Sometimes Goons, Monument, Bishop, Gardens, Duke, Vineyard, Ambassador, Rabble, Saboteur, or Silk Road can totally change the landscape of the game and drag it out too long.
Some strategy article writers make a big point of comparing your strategy to a 13-turn, 4-province baseline. All well and good. But what if I refuse to meet you on the other side? Rabble-Ghost Ship-Monument-Fishing Village. I don't even touch the Provinces. Have fun with your 3-dead-card hands.
I'm also willing to dramatically shorten a game. I've feasted a curse to run the pile out, and I've won games -1 to -7. I don't care if my wins are pretty.
2. "Bad cards" are good more often than we'd like to admit. Some "Good" cards are bad more often than we'd like to admit.
Some of my best cards are practically a who's who of cards that level 30 players have dutifully learned to stay away from or be wary of. Theory's articles [Edit: articles published on DominionStrategy that weren't actually written by Theory] will tell you that Smugglers is a very bad card. This is a white lie to help new players, who fall for it as a trap card. Smugglers is a bad card in 70% of games. In the other 30% - well, you know those games where you get reduced to a sputtering rage because your opponent keeps getting "free" copies of the Wharves and Festivals you "worked so hard for?" Yeah, I'm that guy. Smugglers is one of my three best cards given available. I buy it in 30% of games, but I buy it in the right 30%. I'm similarly strong with Black Market and Outpost, which are also commonly considered traps because they're overrated by new players.
Some of my cards for best Win Rate With? Thief(!) is actually #1, with Stash, Explorer, Bureaucrat, Noble Brigand, and Oracle up there as well. I only rarely gain these cards (3%, 7%, 7%, 6%, 11%, 24%) but I do so in the games where they actually are worthwhile.
My best win rates without: Gardens, Alchemist, Militia, Farmland, Hoard, Smugglers, Silk Road, Secret Chamber, Ghost Ship, Apprentice.
Seven of those are incredibly powerful cards I love to have in my deck in most games. (Smugglers, SC, and Farmland aren't particularly powerful.) The difference between me and a level 30 player is that I buy these powerful cards less often than they do, because I see something that they don't about the shape of the board.
If you see a level 40 player buying Ghost Ship, you should be worried. If you see a level 40 player skipping Ghost Ship, you should be terrified.