Tumblr is the embodiment of Sturgeon's Law. If you stay in your own little corner of the site, contained in the 10% of the site that isn't crap, you're gonna be fine. If you try to branch out at all, though, you're going to be confronted with the 90% of the site that is, in fact, crap. When I tried out Tumblr, I found that I couldn't fit in with most of the "fandoms" that are prevalent there. I mean, I'm an enthusiastic guy, but they take that to a level that is entirely to high for me to be comfortable with. I've also kept an absurdly low profile, meaning that I've never had any run-ins with most of these "Social Justice Warriors". I spend most of my time on Tumblr looking at NSFW images and attempting to maintain my relative invisibility.
A lot of people regard Tumblr as a "safe place", but the problem with that is that they use it as an excuse to spout hate and intolerance, as well as, frankly speaking, poorly thought-out and unjustified assumptions and theories about their favourite characters and works of fiction. Seriously, whenever I see the term "headcanon", it says to me "I'm going to ignore the original writer's intention, or lackthereof, and assign traits to the characters that I want them to have". Speculation is fine, but it should be accepted as mere speculation, and there has to be some justification. I highly doubt these people are the majority, of course, but their lack of prudence gives people the impression that the entire site is full of idiots. The truth is, people gravitate to Tumblr because of its lack of censorship and its advocation of free speech. While in an ideal world, the lack of regulation with make it a wonderful website for everyone, but in practice it lets the less prudent members of the site drown out the quieter members who have actual quality things to say.