-when people check their cellphones. Unless you are a medical doctor on call there is no reason you cannot wait 90 minutes to check your cell. You may even walk out of the theater if it is very important. Do you not realize that when you open that cell-phone in a darkened theater it becomes a beacon of light that everyone behind you is drawn to and thus distracted?
-talking. Some movies demand talking, Snakes on a Plane come to mind. But if you missed a joke or plot point it is unnecessary to ask for an explanation because now you have missed another joke or plot point.
-advertisements before movies. Going to the movies has very few moments to socialize (see above point)so the only times to politely talk is before and after the movie. This is no longer possible because cinemas blast advertisements. It was fine when there was no audio and acceptable when the audio was limited to background noise but it is just wrong to blast those ads in full THX, ear shattering decibel ranges.
-no intermission. I like the bathroom break a DVD provides.
-no rewinding. I am a little OCD with my movies and sometimes like to rewatch a previous scene so I dont miss something.
-too crowded. I rather not sit next to a stranger then I have to sit as compact as possible not to intrude on my neighbor's space.
-theater hoppers. It is OK if you decide to catch a double feature, that is the theater's problem. But when you enter the movie half-way, catch a scene, then leave, you have distracted me.