I feel it is just basic social ettiquette and not 'audience ettiquette' that stops me from opening a packet of boiled sweets in the quiet moments of the 'Ave Maria' in Verdi's OtelloI think one way to steer a middle course between excessive condemnation of audience noise and allowing any and all distractions is to notice that some audience noise indicates enthusiastic enjoyment of the music: this includes things like clapping between movements and shouting "bravo". I really think we should be very careful about suppressing any audience demonstrativeness. On the other hand, there are some kinds of sounds, coughing for example, or the unwrapping of candies, that indicate obliviousness to the music and I think we should speak out about that sort of thing. I have attended hundreds and hundreds of concerts and I have never coughed audibly in a single one--even if it was a bit of a struggle a couple of times. Sometimes audiences seem to forget that they are listening to real human beings and are not in front of their television sets. They just need to be reminded, perhaps.
There are other kinds of concert etiquette that govern the performers and not the audience and these seem to be less talked about. Take the protocols about entering and leaving the stage and bowing. In an orchestral concert most of the musicians come on stage informally and take their places without ceremony. But when the concert is about to begin the concertmaster enters, to applause, and in collaboration with the principal oboist, supervises the tuning of the orchestra. Then the conductor enters, to applause, and usually shakes hands with the concertmaster and principal cellist. If there is a soloist, in the case of a concerto, for example, the soloist and conductor enter together, with the soloist in the lead. The soloist then shakes hands with the concertmaster and perhaps principal cellist. The soloist bows to the audience and after getting settled, indicates to the conductor that he or she is ready to begin.
These protocols signal important facts about who has the musical priority. After the performance, the soloist again leads the bowing and may go ahead and shake hands with the concertmaster and any instrumentalists who had important solo parts. The soloist or the conductor may signal to important soloists to stand and receive applause. All this is done quite naturally and even though it is a pretty strict protocol, it is also quite natural as it corresponds to the musical importance of the parts and roles.
The sequence of bowing by the soloists in an opera is governed by very similar conventions. The minor roles enter and bow first and the last to enter and bow are the major soloists and then the whole cast bows together.
Personally I like the formality of a classical concert as I believe it has the effect of designating the concert hall space as a special place with its own rules and procedures. I think that the traditional protocols tend to protect and enhance the listening experience.
This clip just catches the entrance of soloist Mstislav Rostropovich in a performance of the Shostakovich Cello Concerto no. 1: