I tend toward the pragmatic side when choosing presidential candidates. I don't consider supporting any candidate I doubt has a good chance of winning a general election. Here's my past list of candidates:
2008 - Obama, but was an evangelist for Mark Warner before he dropped out and Obama got in
2004 - John Edwards
2000 - Al Gore
1992 - Bill Clinton
1988 - Paul Simon, but only after Joe Biden dropped out
So in looking toward 2016 (as we Iowa Democratic activists should do, since we do control the choice of who will be the most powerful person in the world), I'm writing off candidates who seem to be difficult to elect in a general.
Now, such calculations can be wrong, of course. For example, for the 2008 caucuses, I was strongly anti-Hillary on the grounds that she'd be another John Kerry -- too easy to caricature as a cold disconnected elitist. However, by the Pennsylvania primary in late spring, she'd turned her public image into that of a shot swigging good ole girl and it was Obama who'd let himself be framed as an aloof elitist. I loved both of them and thought either would make a fine president, but you only get to be a fine president if you can get ELECTED president. And the aloof, elitist card has been one of the most effective that the Republicans use against our candidates (Dukakis, Gore, Kerry, and, less effectively, Obama).
Thinking back over successful Democratic candidates here are some other things that stand out: they tend to be fresh faces in politics, and they tend not to come across as professional politicians (even if they are). For example, Bill Clinton was great at portraying himself as a crusader for the "forgotten middle class." They also tend to have not grown up well off -- which I imagine comes across both in biography and in ability to connect with regular people. I think it's worth noting that the two recent Democratic winners (Clinton and Obama) both grew up the children of single mothers in sometimes difficult family situations. Neither had to fake empathy with people struggling with hard stuff.
For 2016, the candidates I'm inclined to rule out are Martin O'Malley and Andrew Cuomo. They're probably both awesome people, but they come across to me as professional political hacks. I can't see either winning a tough general election.
Much as I like Mark Warner, he's had to be so middle-of-the-road for Virginia political reasons, that I doubt he could get much support in the primaries. Sort of like Joe Biden in 2008 -- would be a fine nominee but why support a guy who isn't a viable option.
Among possible candidates for 2016, that leaves the following: Hillary Clinton, Joe Biden, Amy Klobuchar, Elizabeth Warren, and Sherrod Brown. Maybe there are others considering running that I'm not listing, but no one comes to mind who would get into contention.
I think any of them would be strong general election candidates, so if the race comes down to one of them against one of the others, I'll support which of them is most viable, I suppose.
However, the one I can most imagine doing great electorally is Elizabeth Warren. She seems the one most able to fully hold and then build on the Obama coalition -- bringing her own energy and enthusiasm to the race without losing anyone. Brown's trade populism (with much I FULLY agree) risks losing some of Obama's educated elite support. Clinton and Biden wouldn't have the fresh-face, appeal-to-youth enthusiasm that winning Democrats (Obama, Clinton, Carter, JFK) tend to have to put them over the top. That leaves Klobuchar and Warren.
I'm inclined to Warren because of the crusading energy (like Obama's in 2008) her candidacy would bring to the race. I'm also optimistic that her populist economic message could bringing back some of the working class voters whose economic interests lie with a Democratic agenda but who didn't support Obama because of cultural issues. And unlike Sherrod Brown, I can easily see her bringing these folks in without losing support from educated elites -- since she's the freaking DARLING of the liberal elites, despite her own low-income background.
Klobuchar is probably my second choice -- mostly because of concern that her more consensus-y soft-spoken style risks a sleepy general election campaign for Democrats against Republicans hungry for a return. (I'm assuming a hard-to-beat Republican candidate like Jeb Bush or Chris Christie.)
I have no idea if she'll run, but Warren's my 2016 choice three years before the caucuses.