Here is the the 2012 NFL team average for passing statistics:
338.5/555.9 (60.9%), 3,700.6 yards, 23.7 TDs, 14.6 INTs, 7.1 yards per attempt, 83.8 rating
And here is Joe Flacco's 2012 passing line:
317/531 (59.7%), 3,817 yards, 22 TDs, 10 INTs, 7.2 yards per attempt, 87.7 rating
And this is no outlier season: this is pretty much a typical Flacco season in his career. In each of the past four seasons, he's thrown for 3,610-3,817 yards, 20-25 TDs, with 10-12 INTs. He's remarkably consistently average!
And don't go thinking Flacco has traditionally elevated his game in the playoffs. He's now played in a pretty impressive 12 playoff games--not only have his teams made the playoffs in each of his five seasons, but they've also won a playoff game five straight years. But that hasn't been about Flacco. In "his" first three playoff wins, he had 24 completions combined. He's completed 60% of his passes in just two of those playoff games, and thrown for 200+ yards in just five of them (though he's compiled a lot of yards in his last four, and has looked really good in the 2012 playoffs).
Joe Flacco was fortunate to be drafted by a team that is incredibly well-run (they consistently draft well, constantly restocking the roster with talent), very well-coached (John Harbaugh was a first-year head coach in Flacco's rookie year), and has been traditionally strong at defense (from '08-'11 the Ravens ranked 3rd in points allowed each year--this year they dropped off) and running (Ray Rice was also a rookie Flacco's rookie year). That is precisely the team that can win with an average quarterback. That is precisely the team that can torment its fans for years as it struggles to find an average quarterback (the Ravens did!).
Which is why, even though Joe Flacco is average, the Ravens would not be foolish to sign him to a big contract. It is important and difficult to find a quarterback you can win with in the NFL. It's a problem to credit a team's win-loss record to a QB, but it's not stupid to look at a team's record with a QB at starter. If a QB has a good win-loss record, what it tells me is that team can win with that guy at QB. A team's primary* goal is to win games, not find an elite QB (though finding an elite QB is usually the means to the primary goal), so if it can find a QB it can win with, the team shouldn't let him go. The market probably dictates that you pay a lot for an average QB, especially if that average QB has been on a team that wins a lot of playoff games and makes it to the Super Bowl. And that's not bad, if you've got a well-run organization that consistently drafts well--if the Vikings had average QB play from 2005-2012, the team would have been much, much better off over those years.
*actually the primary goal of the team/franchise is to make money, but that doesn't really help in analyzing and commenting on the game itself terribly much.