Do learn anything from D&D? It is an interesting question, and as I compose my response, I am surprised by the fact that the answer I am currently submitting is different than my gut reaction.
My initial response was yes, it has taught me a lot of things: math skills, organizational skills, social skills, etc. Upon reflection, however, I would have to say no. D&D doesn’t really teach anybody anything. It may reinforce math and social skills, it may even broaden historical knowledge, but it doesn’t actually teach anything.
For example, take mathematical probabilities. I didn’t learn about probabilities from D&D. D&D provided me an entertaining way to apply what I learned in school. It provided me a way to exercise those skills, but it didn’t teach them to me. It probably didn’t teach them to anyone on these boards. Even if you were really young when you started playing, the best it could have done was introduce the concept. Saying that D&D can teach someone probability concepts is kind of like saying that playing Monopoly over and over can teach someone accounting skills.

If one’s mathematical skills are limited to what you use in D&D, then that person has command of only the most basic of math skills (that goes for Monopoly and accounting skills as well

).
D&D doesn’t teach organizational skills. It provides us with an opportunity to practice them, but there was never a chapter in any version of the rulebooks that discussed how to properly use a Franklin Planner.

It was from your family -- or from the classroom -- where you learned to clean your room, put your toys away, to write legibly, etc.
As for social and moral lessons, again, I believe that it can reinforce both good and bad tendencies, but it teaches nothing. It doesn’t teach that working together is generally beneficial. It can reinforce that thought, but all of us were already familiar with the concept long before we ever picked up a d20. It doesn’t make the nonviolent violent, nor the socially withdrawn into social butterflies, but it can reinforce.
Oni was right earlier when he said, “D&D doesn't teach so much as reveal what you brought to the table in the first place.” This true, not only for moral lessons, but for more concrete lessons as well. Actually, this is true of almost any recreational/social activity.
I can see SemperJase that you feel very strongly about your position. Unless, I am just completely missing your point, you are kind of making a nature vs. nurture argument, with you taking a very “nurture” stance. As more of a “nature” supporter, I don’t completely agree with your point of view, however, I would concede that for those of a disturbed nature, playing evil characters – even smart, subtle, sophisticated evil characters – probably isn’t beneficial for them or those they come in contact with.