People who see a thread that ostensibly holds no interest for them and, instead of just ignoring it and scrolling by like a normal person, feel the need to click into the thread and then take the time to ask "Who actually cares about this rubbish?" Well, considering that the vast majority of MN users didn't engage with this thread, but you did - it would appear that YOU care about it.
Those who post a grainy scan picture and ask others to say if they think it's a girl or boy. Answers are invariably 50:50 and surely of no use whatsoever.
Posters who use the same or near identical and specific phrasing as the post above theirs, but don't quote it or say "I agree with this" - they post it as if it's just popped into their head. Sometimes you get three or four near identical posts in a row. Not just standard acronyms or common and predictable responses, but sometimes quite distinct phrasing. It always reminds me of people who, when asked e.g. "Would you say that your family is important to you?", will parrot it back: "I would say that my family is important to me" rather than the more natural "Yes, I certainly would".
That stupid biscuit. People think they're so clever when they smugly reply with nothing but a biscuit. I suppose they think it's an automatic sign of undeniable superiority and rectitude, but I always take it as a clear demonstration that, whilst they actively want to engage with the conversation, they sadly don't possess the verbal skills to be able to explain why they disagree with the OP/PP, much less actually add any further thoughts of their own.
I have RTFT and add my 100% support to those complaining about people who don't RTFT - especially those who actually start their response by confirming it, as if that justifies it. Whenever somebody asks why people don't RTFT before commenting, you always get people who say "But I'm busy; oh, excuse me for not having time to read 16 pages" etc. All they're saying is that their time is more important than anybody else's: The thread is too long for me to be expected to read, but here I am happily making it longer with the same comments as we've already had a hundred times and/or with irrelevant musings now that the thread has moved on and expecting other people to read my comment as well as all the rest. I know it's not the same thing entirely, but I wonder how these people would react if they had a GP appointment, told the doctor their symptoms and concerns and then s/he said "Well, I haven't listened to anything that you've told me, but have you tried losing weight or giving up smoking?"