The most important lesson is failure. It is a lesson that many games have yet to learn.
There is a feeling for people who have been through trauma. Car accidents are probably the most common source of this feeling; the most likely for someone such as an unaware adolescent to experience. It is the experience of a mistake that has been made and cannot be undone. Repaired, perhaps. Forgiven, perhaps. But never erased. Never removed. And people who have experienced that state, whether they were the cause of tragedy or the victim of it, do not easily forget it.
It is the state of failure. It is the state wherein one realizes one's vulnerability, the fragility of one's shell. Rather, it is the state where the childhood feeling of invulnerability is abolished. It is the state where one realizes that things can happen that cannot un-happen. It is the lifted shield of juvenile delusion and the exposure to responsibility and consequence.
And games have yet to experience it, to embrace it. Games fear it because games see themselves as the shield. How can it be their job to create this sensation of weakness when the purpose of games is to escape from responsiblity, to avoid consequence? Games save your data so that when you stumble you can get back up. Games give you safety harnesses and airbags and insurance policies so no matter what tumble you take you can get back up without learning anything from it.
This is fine for games that don't aspire to be anything but a fun, distracting experience.
But now we're making games that try to be grown-up, aren't we? We're making games that aspire to something greater. Games like The Last Of Us, which supposedly tell the story of survivors fighting the odds, with death and loss happening on their journey. These are deaths that are meant to evoke emotion, not simply to be the loss of an AI character. These are deaths we are meant to care about.
But that death happens in a safe place, doesn't it? It happens where the player can't touch it. "It's not my fault" says the player. And if by some miracle a death happens that IS the player's fault, the game shrugs. "Well, we'll just give you another go at that one. That one doesn't count". The threat of death is reduced to an inconvenience for the majority of the game and then they attempt to draw emotion from it when it happens outside of the player's control.
Games like this are the equivalent of an adult giving a child a bike with training wheels, then later shoving them to the ground. "Why didn't you LEARN from that?" they cry, ignoring the fact that there was no logic or reason to it, nothing that could be avoided or improved upon. Games like that control everything and berate the player for doing nothing.
This is not to say that there are not games with consequence and hair-trigger decisions; Hitman: Blood Money is a perfect example. A person playing Hitman morally is forced to make difficult decisions. Every move and mistake could result in needless death of innocents; the Hitman's ultimate goal is to kill his targets with as little collateral as possible. A person trying to be invested in the process is under heavy pressure, then, as a single mistake could lead to a compromised identity, a witness that needs to be eliminated, a team of innocent police officers that must be gunned down. Failure in Hitman carries moral weight to it because it complicates the situation and it has consequences.
It's also not entirely fair to blame games alone for this. Movies have suckled viewers on the concept of consequence-free stories for decades, so much so that films like Chinatown and shows like Game of Thrones stand out simply for not doing that. But what's different between movies and games is that games are interactive, and thus failure can be completely natural rather than forced. And if we care about evoking emotions - that is, emotions other than vicarious triumph and saccharine attachment - then we must look to that interactivity for ways to accomplish it. Consequence is easy, if you shape the game right from the start.
Sometimes we must simply accept that we are fragile, that we can be broken, and that others share this. Sometimes we must learn that the world isn't as safe as we'd like to think. This does not have to be taught through complex symbolism or strained metaphors. Just the realization that every action we do, or do not do, affects the world.
And if gamers don't truly believe in the worlds they inhabit, why on earth would we consider games art?