When people block me for having different opinions or stating my opinions. I mean, really. The ignore list feature is only good if someone is flaming you directly, trolling you directly, harassing you, or spamming/advertising. If I have opinions against tumblr, different political opinions, or have anything in my profile or sig they do not like, those are not good reasons to block.
Case in point, I said that tumblr is sexist because someone told me that feminazis use it, and some girl abusively and unjustly blocked me for saying that. I know that's only some of the tumblrs that are sexist, but what I learned from someone else is that tumblr is mostly female and 4chan is mostly male. I later learned that Tumblr isn't as bad as what I believed, but it's already bad enough that this one girl blocked me for stating my opinion. It's also not ignore list worthy to oppose intolerance or insult someone's interest without the intention to annoy someone.
I don't believe that tumblr is sexist anymore, but I'm still not joining.