The worst thing about flying isn't the recycled air in the planes, or the cramped conditions; it's the other passengers.
I just can't figure out what's so hard about getting on a plane and getting your ass in the seat so other people can get to their seats. But boarding the Emirates plane in Dubai, the two guys in front of me took about 3 minutes to figure out how to get their bags into the overhead bins, even though the bins had no other luggage in them yet, and decide just which seats were theres, then actually get themselves seated and out of the way. And they looked suspiciously like businessmen, which had me wishing I could know which company they were with so I could make sure I didn't have any of their stock.
Then there are those who recline their seats all the way back as soon as the plane takes off. There's nothing like having a seat back 12 inches in front of your face to make a trip unpleasant.
And finally, not so much a pet peeve since it doesn't negatively affect me, I've never figured out why everyone jumps out of their seats and stands up the moment the plane reaches the gate. It's always 5 minutes or more before the door gets opened, and meanwhile everyone's crowded in the aisle, or hunched over underneath the luggage bins. I used to choose aisle seats so I could more effectively stretch my legs to keep my knees from hurting, but then I had the nitwits in the middle and window seats hovering over me and giving me the visual death threats if I didn't get my ass up and stand there doing nothing as well. So I moved to window seats, and now I can just comfortably sit there reading until all the morons have cleared out, then get my bag and exit the plane without trouble.