Mostly it's just the oversized carry-on bags!!!! Oh, and people who don't know how to put their bags in the overhead bin. Wheels first! And if your bag doesn't fit in wheels first and you have to put it in sideways (thus taking up two spots) then your bag is TOO BIG!!!!
There is really not too much that bothers me on flights, but I can narrow it down to a few things... What Twiggers said is one of them.
The other thing that bothers me is when people are sick and contagious on a flight that are coughing, sneezing, etc. It's unfortunate that they are sick, but they really should be wearing a mask on the flight. Sounds extreme, but I do not want my vacation ruined that I paid good money for by getting sick because of someone who is spreading their germs during the flight.
Another is when a flight is delayed enough for me to miss my connecting flight, or a cancelled flight.
The last is lost luggage and the improper way the airlines handle this. My boyfriend's lost bag was dropped off at another woman's house and she was kind enough to contact him. My lost bag was sitting at the airport for days and they said they couldn't find it. I drove to the airport myself and spotted it just sitting in front of their office.