Any and all RPG princesses must either be---
a) Wearing pink
b) Absolutely vital to the plot (this is a very popular one)
c) Captured by an evil Villain (By far the most popular)
d) Rebellious
Examples of this are plainly seen in SMRPG, FF1, FF2, FF3, FF5, FF9, FFT, LoZ, CT, CC, WA, BoF, BoF2, BoF3, L:SSS, L2:EB and many many many more!
Exceptions to this rule---None that I can think of. It's pretty much obeyed all around.
In many RPGs, there exists a mystical pendant, talisman, or necklace that has hidden powers, can open time warps, holds the secret to the villain's weakness, etc. These pendants are most often possessed by the above-mentioned princesses. If not held by a princess, these talismans are always held by a female character.
Examples of this are seen in FF3, FF5, FF6, FF7, FF8, CT, CC, WA, L2:EB and many others.
Exceptions to this rule---Well, not all princesses have pendants, but a lot of them do.
Very frequently, male RPG heroes are mute, or speak very seldomly if at all. No one knows what causes this, but it appears an awful lot. Seen in CT, CC, SoM, LoM, LoZ (all 6 of them), WA, GS, GS2, etc.
Exceptions to this rule---The Final Fantasy series, for one. All the heroes in those games speak. Lots of heroes speak....it's just that a lot of them don't as well.
This law occurs in probably every RPG ever---The Recurring Boss. This boss can be at first unbeatable, and then be defeatable after completing some sidequest, or it can just be an annoying boss that keeps showing up---like Ultros, Gilgamesh, the Turks, Golbez, etc. Also, when beaten, these overconfident bosses will almost always say something like "You're stronger than I thought!!". Examples of this are seen in too many games to mention...
Exceptions to this rule---I sure can't think of any right now!
RPGs looooooooove to throw this one at you---Fetch Quests. Say that on your way to the next town, the bridge is out. This bridge could be fixed at any time, but not before you go on a quest to save someone, find something like Mythril to fix it, etc. This also occurs when you come into a town, and...
"Please help! Johnny and Timmy went into the Dark Evil Woods of Utter Despair and no one but YOU can save them! *sob*" And so, you as the hero are forced to enter wherever these children are and rescue them. Usually, though, you have to save them from an evil boss beforehand.. Not to worry, though---once these quests are completed, you can go on your way! The bridge is conveniently fixed! Imagine that!!!
Exceptions---Nooooooonnne. Nope. Sorry, go save Timmy, no way out of it.