Yes, roles model. That's not a typo.
MMOs and other multiplayer games could learn a great deal from Left 4 Dead, particularly in regard to combat roles. In Left 4 Dead, no person in the group is mysteriously regarded by all enemies as the tank, so all players have to keep an eye on each other.
"Grouping" hardly describes it. This is as close to real soldier camaraderie as I've ever experienced in a game. You have to constantly watch the backs of your buddies. They rely on you completely; they pay for your mistakes... not with minor injuries, but with their lives. You're sharing resources -- whoever needs a healthpack most gets it, regardless of who happens to be carrying one. You can only carry one at a time, so giving is always a sacrifice.
And perhaps that's the key component. More than any game to date, Left 4 Dead enables and encourages sacrifices for the sake of teammates. Sometimes you sacrifice pills or healthpacks. Sometimes you sacrifice your life.
I'm not stretching the truth. Among my friends in Left 4 Dead, it's not uncommon to see true sacrifices -- selfless acceptance of hardship for the sake of another. One person will speak into his headset, "Don't come get me", because he knows the rest of us stand a better chance of surviving if we don't chase his captured and pinned character into the middle of an unmanageable horde. Death means he is only a spectator for minutes at a time... possily a long time. Sometimes the player healing another with his healthpack is down to half-health himself, but he's chosen to help the person with the most need.
It's honestly pretty touching, and a phenomenon all game developers should take note of. With the unlikeliest of settings, a zombie infestation, a game has managed to contribute a deep and lasting lesson to gamers. People can learn more about being a soldier from playing Left 4 Dead than from most war games, and even many documentaries.