Kevin Drum has an interesting take on why macroeconomists can’t seem to agree on even the basics:

Sadly, macroeconomics combines the worst aspects of just about everything. It’s wildly complex. Its fundamental precepts change over time as the basis of the economy changes. Reliable experimental evidence is practically impossible to come by. Even the effect of fairly simple changes to basic quantities—hell, even the direction of the effect—is often contested. And of course, it’s a field that gores just about every ox in existence. So there’s always a ready audience for any result, no matter how strained an interpretation of reality it takes to get it.

Mark Thoma weighs in here. I’d add that, while it’s no panacea, a little more Kindleberger-style economic history may have gone a long way in helping us see, or at least understand, the financial crisis (and macro crises more generally).