The reason L4D2 competitive modes (but not the co-op modes) are so unfriendly to newcomers is because one poor player can very easily lose the game for the whole team and full games can often last 1-2hrs. It's particularly annoying when the new player doesn't have any idea what they are doing and runs off or attacks on their own and doesn't listen or communicate. The new player is just trying to have fun, but what they are doing is inadvertently trolling the rest of their team, hence rage kicks.