Of course it's too early to talk about 2016. Now that we've gotten that out of the way ...
The most interesting dynamic about the presidential race so far is that the Democrats are behaving like Republicans -- and vice versa.
Since 1940, with the arguable exception of Barry Goldwater, Republicans have nominated the guy next in line. Thomas Dewey almost beat Wendell Willkie for the nomination in 1940, so in 1944 -- and 1948 -- it was his turn. Dwight Eisenhower, whom both parties wanted as their nominee, was a special case, given that whole invading-Europe-and-defeating-Hitler thing.
But Richard Nixon had been Ike's vice president in 1960, and in 1968 Republicans believed he had been the victim of John F. Kennedy's stolen election, so they nominated him again. Gerald Ford was Nixon's VP and the sitting president in 1976. Still, Ronald Reagan almost beat him in the primaries, so the next time around the Gipper got a shot.
In 1988, Reagan's VP, George H.W. Bush, had his turn. Bob Dole (Ford's running mate in '76) had almost beaten Bush in '88, so he got the nod in '96. George W. Bush was nominated in 2000, in part because the rank and file felt nostalgic for his dad during the sordid Clinton years. In 2008, John McCain cashed in his runner-up coupon for the nomination. And in 2012, Mitt Romney did likewise.
Meanwhile, Democrats tend to favor outsiders: George McGovern in 1972, Jimmy Carter in 1976, Michael Dukakis in 1988, Bill Clinton in 1992 and Barack Obama in 2008. Two of the three exceptions were former or sitting vice presidents -- Walter Mondale in 1984 and Al Gore in 2000 -- who used their positions to consolidate power. The third was John Kerry, who won the nomination because of a combination of the mistaken belief that he was the Democrats' best shot at beating Bush -- whom Democrats hated more than they loved outsiders -- and Howard Dean's sudden implosion. (Exit polls showed primary voters didn't like Kerry so much as think he was the most electable.)
On the Democratic side for 2016, the two top-tier candidates are both next-in-liners, Hillary Clinton and Joe Biden (stop laughing). Clinton is the indisputable front-runner: She's more popular; she was the runner-up in 2008; she's the dashboard saint of elite feminist groups; and she and her husband have been working the party machinery nonstop while Biden has been, if you believe The Onion, waxing his vintage Trans Am in the White House driveway.
The contrast between the two parties is amazing.