#1: The Electoral College
It's the mother of all gerrymandering schemes, voter suppression on steroids, and a living, breathing repudiation of democracy sitting like a giant tumor in the center of the U.S. Constitution. It's the Electoral College.
Say what you want about Hillary Clinton -- love her, hate her, or apply any other emotion you like -- she won the presidential popular election by three million votes. In a real democracy that means she wins. Open and shut. Hillary Clinton, based on the votes cast by the American people on November 8, 2016, should be President of the United States.
You can have a democratic republic or you can have an Electoral College, but you can't have both. Long live the Electoral College abolition movement! Death to minority rule!
#2: James Comey
As October surprises go, former FBI director James Comey's was about as consequential as they come. His letter(s) to Congressional oversight committees stated: "In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the [Clinton email] investigation. I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation." It predictably touched off a political and media firestorm of epic proportions.
The timing was crucial. Coming 11 days before the November 8th presidential election, it had the effect of reviving a Trump candidacy that appeared dead. The spotlight was back on the totally false but politically effective "Hillary Clinton is a criminal" meme, exactly where the Trump campaign wanted it.
James Comey knew, or should have known, that delivering that letter to Congress would absolutely be tantamount to handing Congressional Republicans a weapon they could use to great effect in the closing days of the campaign. It was and they did.
In the end, the new cache of emails proved as substanceless as the politically manufactured Clinton email scandal itself. The irreversible damage, however, was done.
#3-4: Russian interference and DNC nomination rigging: It's a tie!
There is overwhelming evidence that the Putin government did everything it could think of to influence the outcome of the 2016 U.S. presidential election in Donald Trump's favor. How successful those efforts were is more difficult to quantify. However, from a robust social media program to fake news stories, email hacking, and Wikileaks' public distribution, the Russian intelligence community at Vladimir Putin's urging were giving it all they had, everything they could come up with, and by all reports they are damned proud of it.
Probably of greatest concern to American voters were the attempted intrusions into the individual state voting systems. According to notifications generated by the Department of Homeland Security, 21 states' election systems were targeted, but not necessarily breached, by Russian hackers. If true, that's a major concern going forward.
Of equal significance was the undermining of Bernie Sanders' campaign by the Democratic establishment and specifically the DNC. All of which was painfully borne out in the hacked Podesta emails distributed by Wikileaks and the mind-boggling tales of chicanery former DNC chairwoman Donna Brazile dropped like a daisy cutter on the political establishment earlier this month.
What Brazile said was that Hillary Clinton was running the DNC from the purse strings down. That worked to capture the Democratic nomination -- just barely -- but in the general election it depressed turnout and enthusiasm. In what was a razor-thin victory margin, or no margin at all, for Donald Trump, that enthusiasm deficit was a major factor.