But They Don't Take American Express
Every merchant in the world -- even those living in far-off villages or hidden floating cities cut off from the outside world for centuries, even those who speak different languages or are of an entirely different species -- accepts the same currency.
Law of Productive Gullibility (Ruby Rule)
Whenever anybody comes up to you with a patently ludicrous claim (such as, "I'm not a cat, I'm really an ancient Red Dragon") there's an at least two-thirds chance they're telling the truth. Therefore, it pays to humor everyone you meet. Odds are you'll be glad you did later on.
Wait! That Was A Load-Bearing Boss!
Defeating a dungeon's boss creature will frequently cause the dungeon to collapse, which is nonsensical but does make for thrilling escape scenes.
All legends are 100% accurate. All rumors are entirely factual. All prophecies will come true, and not just someday but almost immediately.
Adam Smith's Revenge
By the end of the game you are renowned everywhere as the Legendary Heroes, every surviving government and authority figure has rallied behind you, the fate of the world is obviously hanging in the balance, and out of nowhere random passers-by give you a pat on the back and heartfelt good luck wishes. However, shopkeepers won't even give you a discount, much less free supplies for the final battle with evil.
Adam Smith's Corollary
No matter how thoroughly devastated the continent/planet/universe is, there's always some shopkeeper who survived the end of the world and sits outside the gates of the villain's castle, selling the most powerful equipment in the game, like nothing ever happened.
Hey, I Know You!
You will accumulate at least three of these obligatory party members:
1. The spunky princess who is rebelling against her (single) royal parent and is in love with the hero.
2. The demure, soft-spoken female mage and healing magic specialist who is not only in love with the hero, but is also the last survivor of an ancient race.
3. The tough-as-nails female warrior who is not in love with the hero (note that this is the only female character in the game who is not in love with the hero and will therefore be indicated as such by having a spectacular scar, a missing eye, cyborg limbs or some other physical deformity -- see The Good, The Bad, And The Ugly Rule)
4. The achingly beautiful gothy swordsman who is riven by inner tragedy.
5. The big, tough, angry guy who, deep down, is a total softy.
6. The grim, selfish mercenary who over the course of the game learns what it means to really care about other people.
7. The weird bonus character who requires a bizarre series of side quests to make them effective (with the ultimate result that no player ever uses this character if it can be avoided.)
8. The nauseatingly cute mascot who is useless in all battles.