What is the metagame? It can mean a lot of different things depending on context, but all of the meanings share the sense that the metagame is not the game itself, but something above and affecting the game as a whole. In a casual game of poker, the metagame could be as simple as one of the players having an exceedingly poor poker face which makes betting that much easier. On a more complex scale, you have things such as the entire Band of Brothers incident on EVE Online, which has been called by some as what amounts to a forum war that was fought out over the space of the game.
That's a Terrible Idea recently had a post regarding the problem of MMOs as "serious" games due to how they interact with the entire concept of the metagame. As the post outlines, you're first cut out from the endgame by the leveling game, which is changing the variables of the game itself, and when you finally reach the end of the curve most of the strategy involves memorizing specific character builds and raid strategies. The metagame, in this case defined as "the process of strategizing and conceptualizing out of the game," thrives on the viability of different strategies and the necessity of discussion. While there are certainly sites devoted to this sort of theoretical work, they frequently involve simply boiling everything down to a single optimal setup. Take a look at the article (and, if needed, the supplementary piece on terminology), as it's interesting for anyone with an affection for the genre and its overall development.