There were a lot of lessons taken away, one of which is that Democrats absolutely have to unite behind their candidate or risk losing their national parks, functioning government, foreign relations, human rights, and more at the hands of a party that has perfected the art of nose-holding at the voting booth.
The more prescient takeaway, though, is that Democrats WILL unite around candidates with genuine progressive/Leftist credentials, and that putting up Neoliberal candidates who only in the eleventh hour will gesture vaguely towards progressive politics will fail catastrophically.
Hillary's failure wasn't the fault of voters not voting for her, it was the fault of her failing to campaign on a sufficiently electrifying platform and the Democratic establishment refusing to push her to endorse those electrifying politics or putting up a VP candidate who could shore up the Leftist base or at least
signal to them that their policies were received and would be represented going forward. Tim Kaine is the perfect embodiment of everything that was wrong with the Hillary nomination. Here is your anointed candidate, having just won a primary race which, while the final tally was firmly in her favor, was nevertheless a hard-fought and brutal race revealing a strong, energized contingent of young voters eager for genuine Leftist, anti-technocrat politics, who took the loss to the Neoliberal embodiment of all that is Technocrat extremely hard and were still regarding with a great deal of skepticism. And whom do you put up to signal to that energized group that they should be excited to lean in and represent the Democrats as a united front? Not Bernie Sanders, not Elizabeth Warren, not Barbara Lee, no. What they put up was a milquetoast white dude whose sole credential, sole calling card, is that he can speak Spanish. Yeah, no thanks.
The onus is on the party to put up a candidate people will want to vote for, will get excited to vote for, and will get so excited they'll go out and get all their friends to go and vote for.