The theatre is not a movie theatre, the theatre is not your living room. The same for the ballet and the seeing the Rockette's at Radio City Music Hall.
It's great that with so many shows and ticket at multiple price points the theatre is available to the masses, unfortunately the masses can be rude and sloppy.
When you go to a show don't wear jeans and sneakers. It looks bad and it is insulting to the performers and the guests that are dressed appropriately. A simple pair of black pants and dark shoes are perfect suitable and they take less room than a pair of low rise faded jeans and a pair of Ugg boots in your suitcase.
Just because they sell food during intermission doesn't mean you should eat during the performance. No matter how quiet you think you are opening your bag of candy (yes, I mean you in the Mezzanine row B, about seat 12 moved down from the higher rows during intermission at the Phantom performance on 2/25) every person around you can hear that crinkling bag and it is distracting and rude. Save the snack for a delightful treat post-show.
If you can't live without texting, tweeting, checking your messages for 90 minutes at a time - don't go. Even if your phone doesn't ring the bright light of the screen is annoying to the guests seated near you. (And this time I am talking to the young woman in the Mezzanine Row A seat 4 Phantom performance on 2/25 - guess what the light from your phone screen was annoying and I could see it every time you pulled it out from your JEANS pocket.) If you find the show so boring or your life outside of the theatre is so important you can't turn your phone completely off and leave it off for the duration of the show - DON'T GO. Save yourself, your parents, your friends the money and sit in a coffee shop sipping a frappa-mocha-latte-chino while they and everyone else that would have been sitting around you enjoy the show.
(As a side note one of the funnier moments at a show has to do with a patron and a cell phone. I can't remember the performance, I think it was "Curtains" but it could have been "Boeing, Boeing, Boeing". A husband and wife were seated across the aisle from me. Based on the jeans, sneakers, and shopping bags they were tourists. During the show the wife's cell phone rang. Of course it was some loud obnoxious ring that wouldn't stop until she fumbled through her purse and turned off her phone. Her husband gave her such a nasty look - he was embarrassed and furious all at the same time. Well don't worry he got served a big piece of humble pie. In about 15 minutes HIS cell phone rang! What a moron - you would think he would have checked his cell phone the minute his wife's phone rang, apparently not. His wife gave him the best smug filled smile I have even seen. It would have been really funny if only they hadn't distracted me from the performance - twice!)"
Even if the venue allows photography - don't do it. Put away the camera, put down the digital camera sit back and enjoy the show and every person sitting around you will enjoy the show as well. The photos and movies you take of the stage are going to look horrible. You don't have professional equipment and the lighting is terrible. If you really want to capture the memory, buy the DVD, buy the program, buy the soundtrack - the quality will surpass anything you can do sitting in your seat. If the venue requests no flash photography, TURN OFF your flash. The bright light is distracting to the guests seated around you and potentially harmful to those on stage. Put away your video camera, your dark and shaky image will not do justice to the spectacular on stage and you will enjoy the show more if you are focused solely on the performance. Your video screen is bright and that coupled with your raised arm are enough to upset even the most mild mannered theatre goer. (This is dedicated to the people sitting in front of us at the Rockette show in Dec.)
If you are lucky enough to see a show treat the event with dignity it deserves. Respect the venue and those around you and everyone will have the opportunity enjoy the performance.