Soft / Hard rule. Characters can get hit with all sorts of attacks (lightning bolts, earthquakes, meteors, etc.) during battle and still be standing, but for purposes of the storyline, they can be killed by a lowly dagger or sword.
Somnia Rule. Characters never have any problems getting to sleep. You can stay at an inn, wake up, and go right back to sleep, or go to sleep in broad daylight.
Sound Sleeper. Characters put to sleep during battle can sleep through
the various sounds of battles, including meteor strikes, summoned dragons,
exploding bombs, and never wake up -- not to mention being actually attacked
and not waking up. In addition, almost all RPG characters either sleep kneeling
or standing up. (Chrono Trigger is the only game where your characters actually
lay down when they're put to sleep)
SST Rule. Flying vehicles are extremely fast; they can cross the planet in a few seconds. Of course, the fact that all the planets are tiny doesn't hurt...
Stahn's Rule. All RPG heroes oversleep, leading to scenes in which they are woken up by another character, usually the main female character (see Tales of Destiny, Terranigma, Chrono Trigger, etc.)
Status Change rule. All bosses are immune to status changes (poison, sleep, etc.) and instant death spells.
Stealing. In a trend began by FF6, nearly every recent RPG has included a spell or ability that lets you steal items from enemies. (FF5+, Wild Arms, BOF3, ToD, etc.)
Sudden Growth. Until recently, all bad guys would always grow much
larger or transform into a different form when you fought them. In most
recent games (FF7, Chrono Trigger, Wild Arms, Suikoden), however, this is
not the case, except on the final boss.
Superweapon Rule. In most recent RPGs, the bad guys have some sort of super weapon (usually a big cannon) that they use to destroy entire towns (Sister Ray, Belcrant, Light of Judgment, etc.). Luckily, they only fire it when the story dictates it, or everybody would be dead by now.
Swiss Cheese Room. Common type of dungeon room in which there are
many pits. Falling in one puts you in a large, emtpy, room with a single
staircase that leads back up to the room with all the pits.
Symmetric Building Law. Almost all castles in games are symmetric, and most towns are as well.
Taking Turns. Combatants will always move one at a time, then sit back and wait patiently while others take their turns. (exception: Suikoden)
Telepathy Rule. Whenever you are giving permission to go through a pass / gate /
whatever, you can go there immediately and they know to let you through, even though you
just got permission a minute ago.
Tellah's rule. Old men usually get killed (Tellah, Bugenhagen, Galuf, etc.)
Temporal Battle Shift. Whenever encountering an unfriendly personage, one usually experiences a psychedelic effect, followed by a transition to a background that does not match where you are standing. (Exceptions: BOF3 and Chrono Trigger)
That's Enough, Sir. All character statistics (HP, level, attack power) have a limit, which is either a power of 2 (256, 512, etc.) or a bunch of 9s (999, 9999, etc.)
Tide Law. Tides never change.
Titanic rule. Whenever the characters get on a ship, it sinks. The
exception is ships that you control, but even these sink frequently.
Too Many Cooks. You can never have more than 3 or 4 characters in your party for a time. For reasons that are never explained (except in Chrono Trigger), you always have to leave the other characters behind.
Training Rule. It used to be that every RPG had a room / building
with people that told you how to play the game (Earthbounds, all the FFs,
etc.). Now usually replaces with a tutorial mode.
Trauma Inns. For sword impalements, dragon attacks, meteor strikes, gunshots, and even death, nothing beats a nice, refreshing, stay at an inn -- guaranteed to cure all your wounds!
True Form. The final boss always has several forms (usually three) that you fight in sequence. The transformation is often accompanied by a message like "______ reveals his true form!"
Unbeatable Boss. Obligatory boss that wipes you out easily, but you
don't lose the game when you die. Generally a major bad guy fought again
later in the game, and often appears in a Deja Vu Dungeon
Unfinished Business. You must defeat all the enemies to get experience/money for killing them. If you kill some of the enemies, and then run away, you get no credit (i.e., experience) for killing the ones you did. Obviously does not apply to action-RPGs.
Universal System Rule. All game worlds (er, except Evermore) have a universal currency system, and a universal language (except FF1). This is despite most worlds having lost cities, remote elf villages, warring kingdoms, obscure islands, etc.
Unlimited Warranty. Weapons and armor never break.
Untamed Wilderness Rule. There are never any roads (paved or otherwise) between towns, even in games like FF7 where you would expect there to be.
Underwater Vacancy Rule. In games with submarines, there is almost nothing of note underwater. (See FF7 and Lufia 2). Usually there is just a cave or two, and a place where you have to dive to get under some shoals.
Vanity Rule. In addition to characters being available to survive floods, lightning bolts, meteor strikes, etc., their clothing, hair, and appearance are never affected.
Vegas Law. Many games have a place for you to gamble away your money
(Suikoden, Final Fantasy 7, Lufia 2). Most of these gambling
games require no skill, but a very few of them do. They are also usually impossible to win, and/or the prizes cost so many "coins" that you could never afford them.
Vegas Law Corrollary. Saving and resettnig negates the difficulty in winning with the above law.
Vehicle Progression Law. Each new vehicle you get allows you to get
to some new place which the designers didn't want you to go to before. Used
to force you to visit locations in the right sequence.
Venus Rule. It is eternally daytime in games (BOF1+2 excepted), which
is weird enough, but it also will suddenly become nighttime during certain
(so named because one day on Venus is as long as 118 Earth days)
Villager Level. Annoying level in strategy-RPGs in which you must defeat the bad guys without harming the villagers that are in your way. (See Shining Force 3, Vandal~Hearts, etc.)
Villainous Disbelief Law. When defeated, all major bad guys are amazed that you beat them and usually make some remark along the lines of "You're stronger than I thought."
Wandering Mercenary. Another stereotypical character in almost every RPG.
A wandering ninja or mercenary that is helping the party, but doesn't really
care about what they are fighting for. Frequently a popular character.
(see Shadow, Magus, Boomerang, etc.). Usually wants revenge on one of the main bad guys (as in Magus or Vincent's case), and rarely talks.
Warm-up Battle. Rather than just have you wander around town talking
to people, many games start with a really easy dungeon or battle (sometimes
a Deja Vu Dungeon). See the bombing mission in FF7, attack
on Narshe in FF6, opening battle against Zoot in Vandal~Hearts.
Waterfall Rule. All waterfalls have caves behind them.
Weapon Specialization Rule. Each character is very limited in the
type of weapons they can use (except in the SaGa games), usually only having
one type (swords, axes, staffs, etc.) that they can use.
Wild Goose Chase. Annoying part in RPGs where you have to chase
some character (sometimes a villain, sometimes an ally) around the globe,
being informed you "just missed" the person at every stop. FF7 and Tales of Destiny are the biggest offenders here.
Xenogears Rule. Whenever a character is unconscious and then wakes up, another character invariably says "You're finally awake", or something to that extent. Named after Xenogears because it happens all the time there.