I'm ok with them filtering "illegal content" or rather I don't think that's a choice that they have and is something to address within the legal system rather then at a social network level.

Abuse of the system? Like they are sending too much traffic/spamming/malware/etc is fine to limit. Threats, allow users to block each-other; I'm fine with restricting people who attempt to circumvent other peoples blocks.

Porn meh; I don't think it's a real issue I think it's great to allow users to filter pornographic content if they don't want to see it.

My issue is when they start getting into filtering out "incorrect" statements, "dangerous" ideas, even "hate speech" (an every expanding category), etc. It's very distressing that Jack Dorsey thinks twitter has a moral responsibility to remove certain viewpoints from discussions and believes that these discussion will shape the future of our society.