Parents who can't be bothered to supervise their children
People who think they're so interesting that they should monopolize the cabin crew and stop them from doing their jobs... especially those who stand in the cockpit for a natter.
People who recline their seats as soon as they get on the plane, and keep it back during meal service.
People who kick the back of my seat, those who use the back of my seat as a support for getting up or moving around the plane.
People who ignore the yellow line in front of baggage reclaim.
People who ignore the part of the announcement saying "we will be boarding rows 1-9 and those with small children first"
People who moan because veggie meals are served first normally to get them out the way, one year someone was so hungry they pretended to be a veggie and took my meal as opposed to waiting 5 minutes, I had a cup a soup on that flight.
Those who will NOT sit down when the plane is taking off, landing, or going though turbulence.
Those who hog the bathroom.
Those who stand in the aisle way to talk to their buddies seated 6 rows away from them.... then giving me snotty looks because I want to get past to use the toilet.
I'm sure there are more but arghhhhhhh!