Already, by midyear, 4 titles have outscored even the top game of last year! True, this area is a little subjective, but sites like metacritic are more quantitative and aggregate, and therefore (hopefully) reflect consumer opinion.
Part of this may be the trend towards bigger budgets each year per game. Companies would rather put more eggs into fewer baskets to up their chances against a relatively small pool of other "big" releases. This has been called the Hollywood Model. Kind of like the "monopolistic competition" I remember learning...
But then again, the "bigger budget" approach is ultimately sabotaging the industry according to Slate.
But to go back to the score-to-sale relationship, does it really matter that much? An earlier article discusses this. It describes review scores as "least important" to conumers, with genre as the most important determining factor. (This is may be one of the reasons why many mainstream games seem to be cut in thematic ruts.) The second factor also describes this trend: whether customers enjoyed a previous version of the game. This is probably ultimately driving the sequels. Many companies are like Ubisoft in this regard.
But this doesn't stop the fact that sales are still down. Maybe we should take the easy route and just blame the economy?