PC games in general: If you value the work you've done, save your progress constantly
I don't think I've lost more than 10 minutes of work in over a decade. :)

Funny, I have sometimes the same feeling after talking to somebody, especially girls..
Back on what game taught me:
Evil characters are morons (Baldur's Gate)
Infantry can destroy tanks without AT weapons (most of WWII RTS games)
A sniper rifle shot in the foot is always deadly (most of FPS games)

Morrowind: You can sneak right up to an enemy in front of him in broad daylight and not be seen.
Fallout 2: You can fail to hit bloody big mutants that are right in your face.
Soldier of Fortune: Headless people can breakdance.
Doom: Barrels are volatile. No exceptions.

KOTOR: doesn't matter what happens to your friends in a firefight. as long as you are breathing they will be alright.
Fallout: Nuking the world will make it awesome
Deus Ex: Sword > guns as long as the sword is shiny and futuristic
Sacrifice: Deities are assholes
GTA: Crime pays... a lot.

All games: English.

This too. :p

The Secret of Monkey Island: never pay more than 20 bucks for a computer game.

Serious Sam series : when you think you cleared a level of its inhabitants, wait, there are more !
The Witcher : Ahhhhh ! Women ! Can't please one without pissing off another !! seriously !!
Heroes of Might and Magic II and III : to try to catch an ennemy hero who has teleport spell is like trying to catch a fly while blind-folded and hands tied in your back...

Dawn of War: Warhammer 40,000: In the grim darkness of the future, EVERYONE is a freaking psychopath.
http://www.youtube.com/watch?v=9gGG4fpGB_w
MDK: The aliens who wish to destroy us all are silly, silly people.
Most Adventure Games: The best way to get ahead in life is to steal everything that isn't nailed down and wreak havoc wherever you go.
Braid: Life is incomprehensible.
Deus Ex: You can't trust anyone who holds public office. Only the people who pull strings from the shadows will tell you the truth.
Final Fantasy X-2: Armor is useless. Women who fight with a ridiculous amount of exposed skin will fare better in combat than people who resemble walking tanks.
Freespace 1/2: There is no mission so vital, nor any operation so dangerous, that it cannot be trusted to a group of eight or fewer pilots of varying skill. Trying to overwhelm the opposing force with superior numbers and firepower is reserved for use by your enemies.
Also, there's really no need to give your fighters significantly better weapons than what the enemy has.
Freespace 1/2: Space military commanders are tightfisted cheapskates.
Freespace 1: Small one-man fighters have relatively little difficulty blowing up kilometer-long capital ships, as long as they can get more ammo.
Freespace 2: Small one-man fighters have incredible difficulty blowing up kilometer-long capital ships unless they're flying in squads of about 20 or they're flying kamikaze.
Freespace 1-2: Technology can change a lot in 30 years.
In Cold Blood: Nuts to stealth. You can just shoot everyone if you're quick enough on the draw.
Earth 21XX series: Post-apocalyptic earth sucks.
Arx Fatalis: You can grow all the fresh vegetables you need underground. Also, barbecued rat-ribs are the best!
Fallout: War. War never changes.
Metal Gear Solid 4: I actually didn't learn anything from this because I fell asleep somewhere during the opening cutscene.

Splinter Cell Chaos Theory: If you try hard enough, your enemy won't see you in front of a white table in all black...in a dim lit room...
GTA 4: I shouldn't drive like I do in that game, otherwise I would have my head thrown out the windshield like 11 cajillion times.
Sniper Elite: Aim low...or high, depending on the gravity.
Star Wars KOTOR: Evil is ALWAYS the way to go.
The Punisher: When in doubt, hold someone over a pool of piranhas.
Far Cry: No matter what, you can blend in with plants even in Red Hawaiian Shirts.
God of War: Threesomes are over rated.
Audiosurf: So this is what tripping on LSD is like...
GTA Chinatown Wars: A friend with weed is a friend indeed.
Resistance Retribution: Brits are bipolar as hell.

In general: Violence is ALWAYS the answer.

You must have never played a Sesame Street game...

Was there one where Big Bird beat Oscar the Grouch to death with his garbage can lid? I think I played that.

We should make that game...