My pet peeve on planes - people who are able to stay within their personal space and refuse to. I can understand that heavy people have problems because the airlines make the seats too small so that they can jam more seats in there. Where I have an issue is when you get someone who can easily fit within their allotted space, but feels the need to extend themselves into your seat because they need elbow room. They have plenty of room in their own seat if they keep their elbows at their sides, but they feel the need to extend their elbows out over the arm rests and into your seat. One of my worst flights involved sitting in the middle seat where I had young business man on one side and this woman with a heavily starched African-style dress with poofy sleeves on my other side. Business man had his elbow in my ribs on one side and the starchy dress lady had her starchy-poofy-dress arm poking me in my arm. I was wearing short sleeves, so her dress was scratching up my arm the whole flight. I got so frustrated I actually wedged by elbows between myself and their arms, said "excuse me," and moved my arms outward to nudge them back into their seats. They each gave me a look and five minutes later they were at it again.
The most annoying flight I was on, from an annoying person standpoint, involved this jerkstore of a male flight attendant who was hitting on two obnoxiously giggly, loud talking college girls. Every 10 minutes he was over there with a drink or snacks or making idle chit chat. Finally he offered them the use of his Game Boy, which they proceeded to play, volume turned up full blast. So I had to deal with bleep-bleep-bloop-bleep-giggle-giggle for the last two hours of the flight.