Huffman is currently holding an AMA to explain his decision. Here's an excerpt from the original post:
Today we're announcing that we're considering a set of additional restrictions on what people can say on Reddit—or at least say on our public pages—in the spirit of our mission.
These types of content are prohibited :
Anything illegal (i.e. things that are actually illegal, such as copyrighted material. Discussing illegal activities, such as drug use, is not illegal)
Publication of someone's private and confidential information
Anything that incites harm or violence against an individual or group of people
Anything that harasses, bullies, or abuses an individual or group of people (these behaviors intimidate others into silence)
Sexually suggestive content featuring minors
There are other types of content that are specifically classified:
Adult content must be flagged as NSFW (Not Safe For Work). Users must opt into seeing NSFW communities. This includes pornography, which is difficult to define, but you know it when you see it.
Similar to NSFW, another type of content that is difficult to define, but you know it when you see it, is the content that violates a common sense of decency. This classification will require a login, must be opted into, will not appear in search results or public listings, and will generate no revenue for Reddit.
 This is basically what we have right now. I'd appreciate your thoughts. A very clear line is important and our language should be precise.
So, as Huffman has said before, "Neither [co-founder Alexis Ohanian] nor I created Reddit to be a bastion of free speech, but rather as a place where open and honest discussion can happen." This policy shift appears to reflects that. Reddit, the self-proclaimed "front page of the internet," will effectively shove all of its vitriolic content as far away from that front page as it can -- just without actually putting a stop to it.
"We'll consider banning subreddits that clearly violate the guidelines in my post--the ones that are illegal or cause harm to others." Huffman explained to a commenter during the AMA, "There are many subreddits whose contents I and many others find offensive, but that alone is not justification for banning." That means communities like /r/rapingwomen will be banned, since they are actively promoting rape while others, like the racist /r/coontown, will be reclassified as NSFW and require additional logins to access.
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