Um. I keep seeing some very corrupted ideas about metagaming...
So I'll lay down firmly what metagaming is, and why I think it should be illegal, or at least very much discouraged as much as practically possible.
Metagaming is when two or more players enter a game of Diplomacy with a prefabricated alliance or hostility. For example, if two friends join a game of Diplomacy and mutually understand that they will never stab each other simply because they knew each other prior to the game and wouldn't want to endanger their RL relationship. It is not metagaming if the two friends enter the game without the intent to stay allied the entire game, but rather would consider stabbing their friend just as much as any other player in the game.
The difference is in the intent and reasoning. If you're reason for attacking or not attacking another player stem from reasons established a priori to the game, then it is metagaming.
Now, this is NOT metagaming...
Metagaming is NOT refusing to stab someone in a game. If you feel they are a worthy ally and do not want to stab them, then that is fine. The problem is when you refuse to stab someone because they're your father's friend, or something like that.
The problem with metagaming? It adds an element of predetermination. With predetermined alliances or hostilities, diplomacy and negotiation are compromised. There is nothing a player can do if two other players refuse to attack each other because of metagame reasons. The player simply cannot offer what the other player can, and the game field therefore initiates in a state of imbalance.
Enforcement is also a large problem with metagaming. Because, like Noodlebug mentioned, it is difficult to police one's thoughts. It must largely be kept with an honour system and the will to play a true and fair game.
While it is incredibly difficult (most would say impossible) to enter a game without any prejudices at all considering it's likely that you will be playing with a player that you've had previous experience with, and the fact that points are displayed as well. Despite this, we should still attempt to minimize our bias, and enter the game open minded.
If it helps, as a habit, I refer to players in my games solely by their countries. In fact, I often completely ignore the names, and as a result, usually forget who I've played with (though I'll always remember several).
Winner takes all is NOT a simple thing. I don't think Noodlebug meant to simplify it, though I cannot speak for him. Though it is based on simple precepts, it's manifestation in game is wide and varied. It is not the bulldog the leader plan that people think of when they think of winner takes all. It is merely a Diplomacy ethical standard. It does not dictate how you attain your goals. It dictates what your goals should be. Never confuse it with simplicity, simplicity is akin to predictability which is the prime vice in Diplomacy.