But this is why I'm doing this meme - to keep my mind off work.
What would make you leave a fandom, or prevent you from getting into it in the first place?
That's an interesting question. My first thought was "toxic fans", but then I realized that I mostly enjoy things with my husband. If I encounter toxicity on the Internet, I simply just don't return, but that doesn't mean I stop being a fan.
Then I thought maybe it's if the content itself tanks, but then I didn't abandon DW in the Moffat years; I just threw myself back into the RTD era and the classic show. As I've noted before, DW is really the one thing that totally captivated my life, so maybe it's not a good example - I don't know if there's anything that could stop me from embracing the parts of the show that I love. I thought back to Heroes which had a brilliant first season and then died the ignoble death of a show which got caught between a writer's strike and showrunners who didn't know the first thing about how to balance a superhero world. (Seriously. They said that they wanted to be totally original so they refused to look at Marvel and DC. This meant that they not only inadvertently copied many of the heroes, but they also copied all of the two companies' mistakes.) Woah, I've really gone off-topic. Anyway, Heroes got bad and we stopped watching it, and basically forgot the later seasons existed - just like "It's too bad they only made one Iron Man movie." ;)
Anyway, I don't seem to leave fandoms if the content starts to suck. I just headcanon it out of memory.
The last thought I had was, maybe I leave when the content creator becomes objectionable in some way... and that still doesn't fit. I'm of course talking about Harry Potter and J.K. Rowling, and to a much lesser extent, Ender's Game (which I really loved but never tried the rest of the books) and Orson Scott Card. I think the thing is, I agree with what Daniel Radcliffe said when all of this started happening, which was (paraphrased): "Don't worry about her. Don't let her attitudes stop you from getting what you get from the stories. What those books said to you, how they enriched your life, that's yours and not hers." I'm a lot more hesitant about buying HP merchandise, but I don't stop myself from enjoying the books and movies.
So, I guess my answer to this question is "nothing".