Some of these have been mentioned above but the most irritating things for me would be
The nastiness directed at MILs and step mothers. If you fall into this category you seem to automatically become one of the most unpleasant people on earth.
People who can't comprehend that other people use the chat forums that aren't parents. The site is designed for parents, however, the chat forums are for everyone.... unless you can only have an interest in gardening, reading, owning a pet, being interested in clothes etc. If you are a parent.
Ageism, in both directions. People who constantly use boomer in an insulting way, while older people constantly bash younger people for how easy they seem to have it.
Spelling, if you get the gist of the post (especially if its a sensitive subject) just let it go.
People who post really unhelpful comments, especially if the situation cant be changed.... the what did you expect to happen comments or the you knew he had children before, why did you choose to have a baby with someone who X, Y Z, why can you just get a better job
And the one that really ticks me off but I'm not sure why, why is everything described as vile???