I want to take a moment to clear something up about Team Fortress 2. There is no I in team. Games are won or lost by the actions of the team. Teams that don't want to defend control points, lose. Teams that want to play defensive classes while on offense, get trapped. Teams that constantly steal ammo packs during setup, thus delaying the upgrade of sentry guns, get run over by early rushes. Teams that play together and support each other, win.
These team mechanics can be both aggravating and rewarding. Even the best solo players can have a screaming fit if their team is not working together. Nothing they can do themselves can make up for a half-dozen or more other players doing nothing. A good player could have an instant kill switch for anything on their screen, but without a team watching their back they will make little difference in the end.
However, a good player can instantly lift a sub par team that is at least trying to work together. A couple good players can almost make a team themselves with the proper classes and strategy. I can't wait to see how more organized clans and teams start doing in tournament level play.
Over all the other aspects of TF2, this is the aspect that sells the game. TF2 would be "just another game" if the developers had submitted to repeated pressure to make each class more "solo friendly". The biggest war was won when the developers stated strongly that grenades for every class were not going to be part of TF2. Grenades would have turned each and every class into a potential powerhouse, capable of soloing any situation. Without the grenades, classes now have to work together to form a team. There are other examples, but the grenades issue was the most apparent in beta and in comparisons to Team Fortress Classic.
Teamwork sells the game.