The postmortems i've been linking to have been focused on policy issues, since i'm interested in seeing the Democratic party rebuild itself after a series of staggering defeats and move away from the DLC centrist philosophy that's been driving it since Bill Clinton. This Politico article focuses on the strategic and tactical mistakes. I don't think there are many lessons to learn from this sort of thing, except in a kind of narrow sense about who to hire to run a campaign next time. But since a lot of Democrats are resisting the need to change, citing the fact that Hillary Clinton won the popular vote, i think this is worth calling out:

But there also were millions approved for transfer from Clinton's campaign for use by the DNC -- which, under a plan devised by Brazile to drum up urban turnout out of fear that Trump would win the popular vote while losing the electoral vote, got dumped into Chicago and New Orleans, far from anywhere that would have made a difference in the election.

Basically, they deliberately ran up the popular votes in places that didn't matter (tactically speaking). I'm 100% in favor of eliminating the electoral college (and not just during the month after a Democrat loses the electoral vote while winning the popular vote), but you can't claim that Trump is somehow illegitimate for winning by the rules everyone understood in advance. Or that everything is fine and we don't need to change anything for next time. You can argue that the people running the Clinton campaign were idiots for taking the Rust Belt for granted while trying to run up the score for bragging rights elsewhere.