My first "real" contribution to F.DS. I generally play 3p on Isotropic without restricting the levels of my opponents, and so I play against beginners plenty. (I'm not excellent but I'm decent - level 30 at present writing.) There are many general principles, discussed on this forum and on places like Dominion Strategy, that beginners can follow to improve their play. But I think concrete illustrations may be a more helpful way to understand them, and to that end I thought I'd flag a couple of common traps with reference to specific cards into which I see beginners fall.
Festival as Silver
This is a big one. If your play consists of playing Festival, playing one other action card, and then buying one card, then you have effectively used the Festival as a Silver - paying $5 for something you could have gotten for $3. If you're not using the extra buy or you're not playing two actions on top of the Festival, then you're not getting mileage and you need to ask yourself whether there wasn't a better $5 or even a better $4 you could have bought instead.
This illustrates two principles nicely. One: think about a purpose for your cards rather than buying lazily on some perceived "value." If you're considering Festival you should think about what other action cards will combo nicely, or how you'll work that extra buy. Two: as people know +actions are classically overrated by beginners (over-buying Villages is such a well-known issue that I'm not listing it here). The way I like to pose the question is, imagine an action card that gives you +1 action, +$2. What do you think that's typically worth? If the answer is more than $3 then you might be overrating the actions.
Quarry on "value"
I think beginners sometimes buy Quarry because they think it's just a good card to have around on value, like a Silver or something. They don't appreciate how severely specialized it is towards action-heavy decks. It's miserable when you need to buy other treasures and victory cards, an extra Copper in your deck. And the money you save often ends up as wasted surplus. For example, you might play your Quarry and have $4 total, and let's say the most expensive action cards are $5. So you have $4 to buy a $3 action - but you could have bought it anyway with Silver instead of Quarry (paying $5 for $5)!
If you buy a Quarry, you should have some clearly specific action combos in mind. Obviously Quarry is much improved with extra buys on the board, but even then you need to be thinking about combos, and specifically combos that are realistically affordable on the budgets you'll have in a given hand. If you play a Quarry with an extra buy but you only have $4 total to spend (not uncommon), you can afford to get two $4 actions, or a $5 and a $3. Do you see such pairs that will work well? Are you sure that buying Quarry to get the discount beats traditional money + full-price?
Envoy on "value"
Envoy is effective when you have lots of extra actions, so that your opponents cannot let you keep actions that you draw, or when you're just pursuing big money, so that even though you lose your best treasure you may still pull an extra $3-$5 worth of money into your hand. If you don't have than extra action and your deck is mixed between action and money, then any action you pull is a dead card that your opponent doesn't have to worry about, and you may end up with all of $1 or even $0 to show for your troubles. Like Quarry, I think Envoy is more specialized than beginners think. You need to really consciously minimize the number of dead cards you're going to draw.
Over-pursuing Gold with Hoard
It's easy to look at the Gold-gaining abilities of Hoard and get too excited. I often see players buying an Estate to gain a Gold or a Duchy to gain a Gold. Think about this as buying two cards worth $1.5 and 0.5 (or 1.5) victory points each. As far as money goes, this is worse for your deck than adding 2 Silvers. You of course receive compensation in the form of victory points - but maybe that won't be enough to keep up with the opponent who is building her deck in a more straightforward manner and will start pulling down Provinces (or worse, Colonies) before you.
Somewhat related is overbuying Tunnels in a Tunnel engine. You get really excited about building a Gold-generating machine and then rush out and buy 4 Tunnels near the start of the game. Meanwhile a more prudent opponent only buys 2 Tunnels and builds up a concentration of money more efficiently. Buy enough to get things going but then stop.
Highway for the cantrip and +$1
On most boards, a card that gives you +1 card, +1 action, and +$1 is not worth $5. (For newbies: a "cantrip" is a +1 card +1 action). Consider that Bazaar gives you that and 1 more action, or that Market gives you that and 1 more buy, or that Oasis requires a discard but only costs $3. If you don't have extra buys and you don't have a card that plays off of lowered costs (ex: Ironworks, Workshop) or extra action plays (ex: Conspirator, Peddler), the cantrip and the $1 in cost savings is all that you're getting. Look at the other $5 cards you could be buying before settling on Highway.
