Other than Jim Webb, there is very little difference between the Democratic candidates. Biden entering the race would not change that much on domestic issues. When he was in the U.S. Senate, Joe Biden put his country before politics on foreign policy. Unfortunately, he has not done so as Vice President but that's because he's part of a team, not leading it. It would be interesting to see if he can contrast with his opponents, his best option is to sell himself as an elder statesman. Hillary Clinton has failed to do so, her strategy appears to be running for Obama's third term, which is risky given half the country disapproves of his job performance.
The Iraq war, the Syria war, TPP, Glass Steagall, fracking, NSA spying, and the like. All very minor issues.
Martin O'Malley and Bernie Sanders are proponents of Glass-Steagall, I'm not sure where the others are on that so I can't comment.
On fracking, Hillary is in favor and Bernie opposes, so yes there's a difference.
On the NSA, the same as fracking, I'm not sure where the others stand.
On Iraq, all oppose the war now, though Hillary voted for it originally. Her and Bernie Sanders have both voted to cut off funding.
On abortion, same-sex marriage, immigration (though Bernie has flip-flopped), economic policy for the most part, Obamacare, cap and trade, the role of the federal government, foreign policy for the most part etc. there are few differences between these Democrats other than Jim Webb.
On the GOP side, you have some similarity. But the field is divided on national security issues, immigration, same-sex marriage, tax policy etc.
