From recent threads I started to think of the different life lessons that each RPG system teaches people. Here is a short list:

Shadowrun: Fight the man, but only if you can make money doing it.
D&D: Killing people and taking their stuff makes me a better person.
Cthulhu: Life is cheap, so why try. Your just going to die anyway.
Battletech: My part in society may be small, but every once and a while I can be a God in my own domain.
Star Wars: Powerless heroes can defeat an all powerful and evil Empire.
Munchkin: Cheat when you can (cause if you don't someone else will).

Here are the truisms for my favorite RPGs:

Shadowrun: Be prepared for every eventuality, and know that what WILL happen will be nothing that you have prepared for. Learn to adapt.
Call of Cthulhu: If you need to use explosives, you can never have enough. EVER. (Oddly, more true in CoC than Shadowrun. Also a truism in the armed forces.)
Tribe 8: In order to free yourself from the confines of ideology you abhor, you must have the fortitude to create your own consistent ethos.
Legend of the Five Rings: It is better to fail honorably than to succeed while compromising your morals. (Correlary: your morals need not be ethical, necessarily.)
D&D: When you get right down to it, money isn't as important as the XP you accrue. But money does help a lot.
Jovian Chronicles: Giant mechs are scientifically stupid, but still totally awesome. And space makes EVERYTHING more awesome.
Iron Kingdoms: Faith, magic, and political power can all be undone by well-placed grenades or bullet. ("Hey, Archduke Ferdinand!")

D&D: "Grave robber" is a viable and profitable profession.

-milk.

Eclipse Phase: Blasting the enemy to atoms is only the beginning of your troubles.

Dark Heresy - always burn the books. (this works in CoC as well).
Delta Green - Crawling Chaos beasts from the great beyond do not have miranda rights.
Rogue Trader - Ya gotta have faith in order to make money.
Shadowrun - if its got stats, you can kill it. addendum: there's always someone bigger than you. Always.
D&D - gamer geeks can have religious holy wars over obscure books written by long dead authors JUST like the 'real' religions do. so choose your ruleset carefully, and be prepared to go to the wall for your decision.
Gamma world - never say die!

Shadowrun: Life sucks because you sold your soul, then you die because someone sold you out, but you will live on because someone sold your body
Battletech: Big equals Big Target equals Big Attention equals Big Trouble

Shadowrun: Literally stabbing people in the back is perfectly acceptable. Metaphorically, it's simply unsporting.
D&D: It's alright to kill them, they're ugly. They're Evil too, but mainly they're ugly.
Exalted: Life sucks until you're on top. Then it sucks more. But you have beer and heroin-pissing dinosaurs, so it doesn't feel so bad.

Legend of the Five Rings:The greatest asset a truely honorable and virtuous man can have when faced with people who do not respect honorable virtue is a loyal low honor sociopath with a penchant for dark pajamas and an aggressive approach to problem solving.

D&D: That all else being equal it's a lot like baseball. When striped of their nostalgia both seem utterly pointless to watch and not a lot of fun to play.

Deadlands: There are fates worse then death.

Shadowrun: Do unto others before they can do unto you.

This is sooo true, both in-game, and historically.

Battletech: In the face of a truly superior enemy with superior technology, your greatest asset is the ferocity of your own citizens. With that you can defeat anyone (or at least hold them off indefinitely).

Shadowrun: No matter how hard your situation may be, there are like-minded fellows.

Rifts: that no matter how hard you strain to avoid it, you will die. Also that death waits for you just on the other side of your power armor. And that it could always be worse in Wormwood.

CP2020: Style before substance
Planescape: If you die, you don't cease to exist you simply move to another plane.
Cadillacs and Dinosaurs: Even after a nuclear holocaust and world-wide energy shortage people will still drive crappy impractical cars.
Vampire the Masquerade: You can always humiliate others by being the host of your own party. Blood anyone?

Indeed!

Delta Force: The question is not about people or money, but time.
Amber: The question is not about money or time, but people
D&D: The question is not about time or people, but money.

Twilight 2000: You can be 18 and still make general before the apocalypse.

Paranoia-Poor Hygiene is Treason
-Fun is Mandatory

Think about it, they're more meaningful in life than in the game, anyway.

Dying Earth: Never fight. It's too dangerous and you don't get anything out of it. A quick wit and fast legs are all you need.

D&D3.5: Being the f'n new guy is okay by others if they consider themselves knowledgeable when you ask questions.
SR4: Still being treated like the FNG when everyone else is an FNG too is not fun and I will no longer game.

Sorry, not humorous but that is really what I took away from it.

Lost RPG: Sometimes the only proper answer is to throw oneself into a flaming jet engine.