I make the point below that Joe Biden would be a tricky choice for Obama because he supported the Iraq war resolution (see comments for some good debate about that). Still, when people talk about Biden they generally cite his foreign affairs expertise as his main selling point. But if Obama is going to choose someone who backed the war, I'd say it's Biden's touch on domestic issues that just might outweigh the thorny Iraq question.
As Nate Silver notes today, Obama needs to convince more voters that he "gets" middle-class economic pain. It's clear his campaign understands this; Obama's two latest ads--one positive and one negative--focus on the economy. Well, I'm not sure there is a single politician in America who speaks more credibly and passionately about middle-class anxieties than Joe Biden. The guy absolutely kills on this subject. (John Edwards, what's left of him, doesn't hold a candle.) Remember, Biden himself is of humble Catholic origins and one of the Senate's few non-millionaires. I'll never forget his 2001 Senate-floor broadside against the Bush tax cuts, for instance, which was one of the most stirring Senate speeches I've seen. I also found him quite impressive on the stump in Iowa last December. The more I think about it, the more I reckon Obama could do a lot worse. (It's also interesting how conspicuously quiet and low-profile Biden's been lately, isn't it? He's the anti-Kaine....)
It's still a whole lot of "Washington experience" to explain away....