The same things that annoy me on buses and trains. I hate it when people decide to have loud mobile phone conversations mid-flight, people talking loudly, babies crying, sleeping people snoring, hearing the music on other people's ipods... just about any noise really. Or when people try to talk me e.g., "Oh that looks like an interesting book, what are you reading?" I generally reply with a snarl.

sitting next to overweight people (no offense but i'm sure you understand).
sitting near groups of friends
sitting near babys or children

i dont know if its an urban myth or wether its true, but supposedly airlines reduce the amount of oxygen in the planes to fatigue theyre customers, well its understandable i guess.
heh i dont really mind flying, i just hate sitting next to people and breathing dirty air.

What annoys me most on a flight? Where to begin?

I'd say the lack of legroom, I'm 6'6", and on some flights, I simply can't sit in the seat. There's not enough room for my legs.

Oh, and the people, be it children or adults, or keep poking and prodding at the back of my seat, Don't know how many times I've had to turn around and tell 'em to knock it off!

And my final pet hate on flights, the person who spills over into your seat. "I'm sorry I'm fat" is not an answer...

Originally posted by neonitus
i dont know if its an urban myth or wether its true, but supposedly airlines reduce the amount of oxygen in the planes to fatigue theyre customers

Pretty much an urban myth, I know of very few cases where pilots have done it, but in most of those cases it was either to calm a group of unruly passengers, or one occasion where a pilot had to dump some of the air to clear the cabin of... Non-Tobacco smoke (back when smoking was still legal on planes). Apparently the air filters weren't even able to keep up with it.