Disclaimer: I voted for Sanders in the primaries and Clinton in the general election.
The following is of course all based on my perception of events, so take it with a grain of salt, but:
Even with an even playing field I do not think Sanders would have won the primaries. However, if the playing field had been more even, I don’t think there would have been as much bad blood from Sanders supporters.
I also think this was the first election that a lot of people really paid close enough attention to (or the news gave as much coverage of) superdelegates. I think it came as a sort of surprise to folks who hadn’t given it much thought before and stunk of back room deck stacking. Why would my vote count so much less than some party hack? And why is some bunch of party busy bodies even limiting my choices of who to vote for in the first place?
Of course, in theory, the superdelegates were going to vote for the majority popular vote winner. But the news cycle kept showing graphics of all the superdelegates tacked up on Clinton’s side long before the primaries, which played right into the idea of Sanders as an underdog, fighting a system stacked against him.
This, and other rumors of inner party shenanigans, and consistent intimations that Clinton was the party anointed one, really embittered both Sanders supporters and anyone else who felt the current system was locked in place and failing them.
I can’t say for sure it lost Clinton the general election, but I don’t think the appearance of unfairness and exclusivity during the primaries helped at all.