[Attention conservation notice: I can’t imagine what you would get out of this if you are not a member of or interested in joining rationalist tumblr.]
[Note: Tumblr rationalists, feel free to comment with anything I’ve missed!]
Absence of rationality content. I’m not going to say that no one posts rationality content on Rationalist Tumblr– bad brains blogging is sort of applied rationality, sinesalvetorem and theunitofcaring have done great posts about effective altruism, and I keep having to scroll past these absolutely incomprehensible arguments about statistics. However, in general, the rationality content is a relatively low percentage, and most of what you’ll encounter is puns, memes, Hamilton, non-Hamilton fandoms, social justice, anti social justice, vagueblogging about whatever one has chosen to hateread, whining about one’s life troubles, and Thus Spoke Carly Rae. In fact, I probably have more rationalist-relevant discussions on Facebook because of people happening to post statuses about rationality than I do on rationalist tumblr.
This seems to really throw people who are used to LW Facebook, r/rational, Slate Star Codex, or other places where one can expect the majority of the discussion to be vaguely related to the alleged topic. I personally like it, because I feel like it makes discussions friendlier if you know the whole person rather than just arguing with them about philosophy. But if this isn’t to your taste then probably you should try somewhere else in the LW Diaspora.
Dogpiling. Rationalists as a whole have a tendency to offer their opinions any time they think they have something remotely relevant to say. They do not have a tendency to check whether fourteen other people have said the same thing. This means that a lot of people’s first introduction to rationalist tumblr is twenty people attempting to explain to them why they should choose dust specks over torture, which is not a very welcoming introduction. On the other hand, it happens to everyone, it does not mean you are disliked, and in fact it probably means people like you and want to be your friend. Otherwise they would just roll their eyes and ignore your post.
Reaction to criticism. As best as I can tell, LW Tumblr tends to collectively view rationality not as a set of beliefs (i.e. Bayesianism is a useful model of human inference, AI is one of the most important existential risks, Eliezer Yudkowsky is always right) but as a set of topics (i.e. Bayesianism, Friendly AI, Eliezer Yudkowsky). So if you post something like “I hate Eliezer, the Singularity is the rapture for nerds, and effective altruism is imperialism and we should have full communism instead”, rationalist Tumblr as a whole will conclude that you are a rationalist and trying to be friends, and will behave accordingly. (“Behaving accordingly” involves reblogging you with long-winded rebuttals; see dogpiling.) Rationalist Tumblr tends to parse statements that are probably intended to be “boo rationalists” as being serious critiques: it tends to go “hmm, maybe the Singularity is the rapture for nerds… well, on one hand, your point is valid because it is an eschatology that’s most popular among Silicon Valley types, but on the object level the Singularity does seem more plausible than the Rapture, and Singulatarians tend to be explicitly universalist…”
Rationalist Tumblr’s exciting friend-making strategy almost never actually works, because apparently most people who write comments about how much a social group sucks are not interested in becoming a member of that group. I have absolutely no idea how we could have predicted this.
I think a lot of people who criticize rationality don’t really understand what is going on because, to be fair, that is a really weird way for an ideology-based social group to work. (Imagine Christians telling Dawkins “you hate Christianity! You’re basically a Christian!”) However, no one has managed to say something sufficiently hateful that rationalist Tumblr, as a whole, has not parsed it as a good-faith critique from a friend that should be considered earnestly. If you don’t want a bunch of rationalists tediously attempting to be friends with you, the only course I’ve found is shutting up about rationality.
Bonobo rationalists. A disproportionate number of Tumblr rationalists are what has sometimes been called ‘bonobo rationalists’, a cluster characterized by polyamory, queerness, being affectionate with each other, the word ‘cute’, Niceness, and a passionate affinity for cuddles. We do have monogamous, celibate, and aro or ace members, and as far as I am aware no one has been a dick to them or deliberately made them feel excluded; however, bonobo rationalist culture can be really off-putting to some people, and it would be surprising to me if no non-poly/non-cuddly/non-Nice people felt alienated. On the other hand, if this sounds like your kind of thing, maybe get a tumblr.
Absence of the alt-right. Despite all the complaints, as best as I can tell there’s one member of the alt-right one is likely to encounter in the course of a normal rationalist-tumblr experience, which is Nydwracu, and most of the content you’ll see from him is him flirting adorably with his boyfriend.
Hamilton. No, we will not shut up about Hamilton. On the other hand, it is legitimately great. Seriously, take two hours out of your life, pull up Hamilton on YouTube, and follow along on Rap Genius. You’ll be glad you did.
Joining rationalist tumblr. Lots of people are nervous about trying to join a new social group. However, we are a bunch of warm, friendly autistics here, and we are not particularly scary. The best strategy to join the group is just to get a Tumblr and start reblogging people; don’t worry if your contributions feel kind of stupid, everyone feels that way sometimes. Remember that we’re a friendly, inclusive group of people who don’t solely talk about rationality: your puns, selfies, or stupid questions that prompt an interesting discussion can also make you a valued member of the community. Many people have joined the community via talking to sinesalvatorem, who is very friendly; I recommend this as a starting point.
I understand that there is an IRC channel, but I don’t participate in it, so I cannot provide advice about that.