I think people should  not be posting  meta on AO3 - even their own - if is talking  about other fans or about what other fans have said or written. I've heard some fans say: well if I am responding to something someone  wrote or posted publicly, I can just quote the snippet without naming them.  But that is WRONG. That is stealing. So if  your choices are 1.  linking  to someone's post without permission or 2. quoting  without permission - your meta does not belong  on AO3.  Besides, how are you going to 'prove' you have permission?  And just because you wrote meta in response to someone else's meta and posted it to Tumblr, Twitter or Livejournal and got away with it for years, does not make it better. It is always wrong to comment on someone else's posts or to link to other people's posts without permission. 

Meta about the canon/TV shows is fine. Just be certain every word you write is your own.