- Cliquish structure.
- Ideas that a "true X" has to hold.
- Specialized language and terms that are completely opaque to non-initiates.
- Special terms for dismissing outsiders.
- Hostility toward outsiders unless they've "proved" themselves.
- Reluctance to consider criticism, especially external.
- Responding to criticism by attacking the critic instead of what they're saying.
- Popular Big Names who few dare contradict.
- Certain facts "everyone knows".
- Double standards for people on the "inside" vs those on the "outside".
- Tendency to put everyone in categories, and evaluate their statements based on the category they're perceived to be in.
- Reactions to things are usually based on emotion instead of logic.
- Insistence that people who disagree with one clearly cannot understand the subject.
Now, am I describing Social Justice, or a fandom?
For more, see the Geek Social Fallacies. These facts often apply to Social Justice in general, but Tumblr has a particularly intense form of it, since the mechanisms to spread information are the exact same ones one uses for fandom.