1. Make as much contrast as possible. Be it a major key, minor key, loud, soft, monophony or polyphony, I believe that contrast is the single most important essence to a good quartet. A good quartet needs a lot of contrast, to keep the quartet interesting to the ear and creates variety in sound. No one instrument should overpower the others in the melody and relegate the other instruments to a form of accompaniment for the entire piece. Playing some of the accompaniment parts personally before, it can be rather dull for them and it creates a dull effect where there's not much dialogue or antiphony between the instruments. One often overlooked way to create contrast is through the texture of the quartet. However, I disagree with the notion that counterpoint should be used at all times and to avoid parallel motion or chorale-like melodies at all costs. While counterpoint is very good at creating unique quartets and is key to a good quartet, when used correctly, parallel motion or monophonic themes across all instruments can elevate a quartet to another level (especially in a climax of a piece). It should be used sporadically at key moments during a piece, to highlight a certain motif or theme, or contrast an otherwise fully polyphonic section, one example of this brilliantly done is the opening of Beethoven's Grosse Fugue. There are many ways to create contrast, and after studying many quartets I realised that every composer has a unique way of creating that sense of contrast in their pieces. It's important to find one's preferred method to develop contrasts in their quartet.