When trying to identify if behavior is toxic, I think it's important to really focus on the idea of toxicity. Being toxic means you are negatively impacting the ways others engage with a medium.
@Celebrim , for example, if you say "I don't get Planescape, it's not for me." That's just an opinion, nothing toxic about it.
You can even say, "I don't like Planescape, and I wish WotC were releasing Greyhawk instead." Again, nothing toxic there.
But crossing over into toxicity would be something like saying, "I don't like Planescape, and those who do aren't real D&D fans."
Or "I don't like Planescape, so I'm going to tweet mean things about its creators."
Or on the other side: "I like Planescape, and if you don't, it means you're a grognard."
Toxic fandom doesn't mean criticism or adoration of a medium. To me, it means poisoning the community and ruining the experience of producing, enjoying, or critiquing art for others.