Here are just a few:

• The leaked mails aren’t condemning per se, unlike some of the media overreactions, but does shows many anti-sanders bias. Here are some: https://www.washingtonpost.com/news/the-fix/wp/2016/07/24/here-are-the-latest-most-damaging-things-in-the-dncs-leaked-emails/
• Superdelegate system is designed to support the establishment candidate against grassroot populist candidates. It has been increasing from 14% in 1984 to 20% in 2008. Before primary was even open, hundreds of superdelegates already pledged for Hillary. “@D_Born @BernieSanders Super delegates don’t “represent people” I’m not elected by anyone. I’ll do what I think is right for the country” ~ Howard Dean (@GovHowardDean) March 5, 2016. Read http://usuncut.com/politics/howard-dean-superdelegates-tweet/, https://en.wikipedia.org/wiki/Superdelegate, and https://newrepublic.com/article/129707/superdelegates-really-stop-bernie-sanders .
• DNC suddenly reducing the number of debates to 6 debates from the 20 in 2008, and suddenly forbidding any more debates. This benefit establishment candidates w/ positive media coverage, and not lesser known grassroot activists/candidates with a message.
• Closed primary that Schultz fully supported ensured that Sanders, with his broad appeal to democrats and independents alike, will not be able to benefit as much from his broad voters base.
• DNC easily over reacted and denied voters access data to Sander’s campaign. http://www.cnn.com/2015/12/18/politics/bernie-sanders-campaign-dnc-suspension/
• The DNC worked with Clinton’s staffs and are very willing to criticize Sanders with the media, but showed a bias against Sanders. http://usuncut.com/politics/dnc-leaks-9-emails/
• The media (not the DNC) gaslighting of Sanders. http://www.dailykos.com/story/2016/4/8/1512595/-A-case-study-in-gaslighting
• The non-conclusive but strange exit poll discrepancy that favored Clinton and showed a possible fraud: http://www.snopes.com/stanford-study-proves-election-fraud-through-exit-poll-discrepancies/

http://www.commondreams.org/views/2016/05/20/establishment-collectively-stunned-see-citizens-reject-rigged-democratic-primary

Anyway, with the really close race, Sanders would have likely won if his campaign have not been harmed by some or all of the above. There are a lot of overreaction in the media and excessive misrepresentation that made it hard to filter through the truth, but judge yourself on whether these are valid concerns or not. I personally hope Trump doesn’t win because of this. Sanders would have truly unified the party, even if he probably wouldn’t have attracted as much corporate donation.

We currently have a system that’ll continue to fortify itself against any non-establishment candidate. The increasing number of superdelegate ensured that.