1) Character: What kind of person makes it through an Ivy League school, then joins the United States Marine Corps? One who believes in service, in causes greater than their own. Love him or more likely hate him for briefly opposing Nancy Pelosi, but at the very least he seems more courageous than most in Congress. Furthermore, as a Marine, he was highly touted not just for his combat skills, but his intellect and decency as well. Not just a warrior, but a warrior-philosopher.
2) Leadership. Speaking of Nancy Pelosi, remember all those swing-district Democrats that promised their constituents that they'd oppose Nancy Pelosi? Most will get to run for reelection in 2020 having stayed faithful to those promises. Agree or disagree with the wisdom of such a promise, but their life has been made easier, as has our path to retaining a Congressional majority in 2020. What did Seth Moulton get in 2018? Unwarranted accusations of sexism and ageism( that were nonexistent in 2016 when the anti-Pelosi Dems tried the same thing), certainly no real reward for his efforts. Yet his actions certainly gave a lifeline to the House Dems with the toughest reelection races, and has encouraged Nancy Pelosi to begin some much needed reforms.
3) Coalition-building. Assuming we don't have a reprise of a Jill Stein type running from the left/Moscow while psychometrics get weaponized against us, Seth Moulton in my opinion possesses the highest electoral upside. How many other Democrats can walk into a veterans hall with the gravitas among vets that he has? I can't predict turnout models, but I can tell you that if we can chip Trump's military support, and get more disaffected Republicans(as Obama managed to do did in 2008), I think a 10 point win is possible. That's a Democratic landslide beyond 2008 and those coattails may help maximize our state and local opportunities for the Census, gerrymandering, and voting laws.
4) Contrast versus Donald Trump. Like everyone here, I can think of a few adjectives for President Bone Spur. But how best to emphasize the Orange Buffon's cowardice? How best to make his fragile masculinity feel threatened? Donald Trump will talk cheap about his love for the country, maybe hug a flag. Seth Moulton need not state what he's done for his country.
5) Healing the country. If you are like me, you're worried like heck about the future of politics(and more...) in this country. I'm not saying that I expect post-Trump America to dissolve into civil war, but it can't be ignored. The right have most of the guns and the left is concentrated in easily-controlled population centers. Even if you're not close to that level of concerned, and even if you want to make the GOP pay for what they've done, we need someone that can at least stop the growing partisan divide. Let's make America sane again. Let's make America civil again.
Go ahead, laugh. Claim he has zero chance. Throw the most recent polling data at me. It's far too early for anyone to know much and with the way this primary is shaping up, it's going to be crowded enough that anything is possible.