I am constantly appalled at people's lack of manners!
I was recently on a flight from Phoenix, AZ to Orlando, FL. I was on Southwest Airlines, got to the airport early, and managed to get myself a nice exit row seat. After takeoff, I hear noise behind me. Sounds like a movie...yep, it was. Someone was watching a movie on their portable DVD player and not using headphones. I turned around and asked them nicely if they could turn it down. Apparently, they could, but it was difficult, painful and quite the bother - at least if the expression on their face was an accurate indication.
I fell asleep, and when I woke up, it seemed to be back to it's previous volume. I was going to ask the flight attendant to say something, but the movie ended or they got tired of watching it by the time I got up. (I had to use the lav and was waiting on the lady sitting in the middle seat next to me to get back so I didn't have to make the gentleman on the aisle get up more than he really needed too. Now that's courtesy!)
Would people consider listening to music on a plane without headphones? I doubt it. And if they did, they would probably be told by the flight attendant to turn it off. Why should a movie be any different? I did actually ask the flight attendant (out of curiousity) if Southwest has a policy against watching movies without headphones. She didn't know for sure, but she didn't think so. That, to me, is amazing.
I wouldn't think a movie being played aloud is something I should even have to a) say something to someone about, and b) wait for the flight attendant to say anything.
People are just rude.