Playing D&D will really teach you the basics of life.
When playing D&D a single player without a party to back him up is quickly eaten alive by the Goblins, Dragons, and Tax Collectors of the world. Only a full adventuring party is equipped to take on any foe.
In the real world a person without any friends or support. Someone who is truly alone in the world, is quickly eaten alive by depression, girls, and tax collectors. (Tax collectors are dangerous in every world).
The importance of NPCs
In D&D NPCs are the people who do all the mundane things of the world. From washing your horse, to planting corn, to being your lover. Most people in the D&D world are NPCs and will never go on a great adventure. If you treat an NPC badly they may steal your horse, poison your mead, or even GASP, stop being your lover. No one can get far without the support of the NPCs
Most people in the real world are NPCs also. Mundane people who lead mundane lives. But if you treat them wrong they might, key your car, spit in your beer, or GASP, stop being your lover. (Once again some things are truly universal).
The Importance of Equipment
In D&D even the most powerful wizard was helpless if someone stole his spellbook, (he could no longer memorize his spells without it). The most mighty warrior was a pitiful sight without a sword or armor. Even the pious Cleric needed a Holy Symbol to do her magic. Without equipment your character was nothing. With quality equipment you could do anything.
In the real world the most elite kernel hacker is helpless without at least a terminal. The most fearless cop on the beat has no authority without his badge, gun, and police car. Even the most mighty waitress cannot take orders without a pen. With no possessions you quickly find yourself out on the street. Which is a hard place for even the most mighty waitress to be.
Knowing Your Enemy
In D&D you had to know your enemies weakness if you ever hoped to defeat them. Otherwise you would quickly die attempting to kill a skeleton with arrows, (they go right through the empty spots between the bones).
In the real world you must also know your enemy. Even if your enemy is only an annoying drunk, or perhaps just a telemarketer. If you know your enemy's weakness, they will quickly fall before you. Otherwise they will quickly defeat you, (or sell you new siding that you don't want).
Knowing when to run
In D&D you would sometimes encounter an opponent that you had no hopes of defeating at your skill level. If you were not smart enough to run, you would be quickly overtaken by the Vampire, Dragon, or Werewolf. Players who didn't know when to run would rarely survive long enough to become powerful enough to defeat these foes.
People in the real world have the hardest time with this. They will attempt to defeat a myriad of girls, bosses, slot machines, lottery tickets, and police officers. Very often they know that they cannot win. But you can retreat, and live to fight another day.
In D&D the average adventurer would spend much of his spare time learning new skills and honing his existing ones, (in order to become more powerfull). The adventurer who never trained would quickly fall behind his peers
In the real world those who spend their spare time training will quickly become more powerfull. In time they will be able to defeat even hardened girls, telemarketers, and pointy haired bosses.