Here's a list I made after seeing a show and discovering just how annoying people can be at a theater.
1) If you are going to be eating candy, UNWRAP ALL YOUR WRAPPERS BEFORE THE SHOW STARTS OR DURING INTERMISSION
2) If you are consuming hard candy, DON'T BITE IT
AND DON'T MAKE SLURPY NOISES
3) Never wear your hair frizzy -- use hair gel if you must.
4) Don't jingle your car keys.
5) If you're chewing gum, don't blow bubbles.
6) Knowing the songs does not give you permission to sing along.
7) Be very careful to not kick the chair of the person in front of you.
8) Eat before the show so your stomach isn't growling during it. Actually, I've never been distracted by someone's stomach growling, but I know mine was once and I recall it being very embarassing.
9) If you have a drink with you -- DON'T JINGLE THE ICE CUBES IN YOUR CUP DURING THE SHOW
10) If you have a cough, bring a bottle of water with you (if you are allowed) and when you take a drink, don't swish the water around in your mouth and try not to swallow loud.
11) If you have a runny nose, keep tissues with you so you don't have to keep sniffing.