The anonymity itself is likely what provides this interesting cultural difference between 4chan and other sites like Tumblr, Facebook, Reddit, etc.

There's no reblogging, liking or upvoting, so there's no real incentive to form what you're going to say so that it lines up with what people would generally approve of. As much as Reddit likes to say that the downvote button is not a disagree button, to 99% of the user base, it completely is. Say you had an opinion on a subject. Hypothetically, this is an opinion shared by less than half of Reddit in general. Logically, one would assume that, in general, voicing this opinion on Reddit would lead to a negative karma score, because you'd be getting more downvotes than upvotes. This in itself would alter many peoples' thoughts about posting controversial things in the first place; if it's going to get downvoted, why bother? Get downvotes, your post will float down to the bottom of the page. Get more downvotes, the post is hidden as if it were spam. Get even more and Reddit won't let you post for a while. This website, regardless of intention, is designed to punish contrarianism and debate. On 4chan, what you say cannot be judged in any way, other than through replies from other people, whose posts have an equally nonexistant weight based on popular opinion. Every single post is held at exactly the same value as far as the site itself is concerned, so only the users, through discussion, can manipulate that. Through arguing. Through arguing. Through saying 'no, fuck you people, that movie isn't as good as you all keep saying, here's why'. It could be a completely flawed point, and if it is, other people will point it out. But it won't be hidden. Nothing is being hidden because it's stupid or off topic or antagonistic, and nothing is being promoted because it's the generally accepted opinion. People will read it, and read other people either agreeing or disagreeing with it, and use their own brains to determine what they think, without letting majority rules dictate what they should be seeing at the top.

You're also not bound to an identity. Sure, Reddit and Tumblr use screen names that don't need to reflect your real life identity, but it still acts as a sort of 'display' for everything you say. We behave differently, at least subconsciously, when we have a name tied to our post, and a kind of 'reputation' we imagine to be attached to that name. Everyone is faceless on 4chan, and nothing you have posted in the past has any affect on what you're posting right now. I don't need to be worried that someone is going to remember my name from somewhere and read my post with a preconceived perception based on what he thinks of some unrelated thing I said a month ago. Just like how everyone's posts are held at equal weight, the posters themselves are indistinguishable from one another. Again, this is on a subconscious level, but the way you take in something someone says will very likely be altered in some way if you know something about them. Some boards on 4chan show a little flag that tells you what country the poster is from next to their post. On these boards, ad hominem exists. Someone will probably dismiss what they have to say because they're a 'typical uninformed American' or not take them seriously because 'Australians always shitpost', etc. On the boards where this feature isn't included, everyone is effectively identical, and information about the poster is only known if that poster makes it known, which generally means that it's relevant to the post.

Anyway. I guess what I'm saying is that a community that has been running for years with anonymity as a core feature to set it apart from other major communities is going to be more brash. More willing to disagree with itself. More willing to say 'fuck being politically correct' and call someone out on their bullshit. Sometimes being blunt and unapologetic is the only way to get a point across, and sometimes wearing a mask is all it takes for you to get over your (and other peoples') inhibitions and just do or say whatever you want to do or say. 4chan is a place where this kind of procedure isn't discouraged because it might be mean and get them downvotes. It's what comes to mind when I think of terms like 'raw' and 'uncensored'.